5Shiiws saiyvyan libraries Smithsonian institution NoumiiSNi nvinoshiiins saiyvyan librari

co z , w z * in z co z

ISONIAN INSTITUTION NOIlfllllSNI NVINOSH1IINS S3IHVyan LIBRARIES SMITHSONIAN INSTITUTION NOimillS
- co ^ z \ co v- — . 5r co — co

* H z H 1

V? -t ?

s ^38^ ^ *j A,, z $80^ <q - Jfk. ^

<

C£

Q 's^VOS*ii>' ^ X>VA^ o ' ^ X^tVAStji? Q \WtQ>Wy — Q

)SHllWS^S3iavyan_,LIBRARIES SMITHSONIAN INSTITUTION NOIlDlllSNI^NVINOSHil WS I y VB 3 ll^L I B R A R I

&X o m X^i^X - . vm 2 m -A* 9 ^

CO m xg» D-c^ ‘■•^5^ m '^2LE^ m ’WL *g m

Z CO „ CO _ CO X “ CO

HSONIAN ~ INSTITUTION NOIlfUllSNI NVIN0SH1MS SBIBVaan LIBRARIES SMITHSONIAN INSTITUTION NOlinilJLS
2 ' 1 co z „ ^ co z in z

w\ < x^vc E .•= „ V= ,.A'v - ^ .As

> Wr 2 .2 -\ > xgugsgx s

w Z CO z CO z CO

3SHHWS S3 1 y vy a n libraries Smithsonian institution NoumiiSNi nvinoshiiins saiyvyan

CO in zz _ — . <o = co __ 5

LIBRARI

Q V^v. Dl^ Q

HSONIAN”* INSTITUTION*- NO I inillSN I "^NVINOSHIIWS^S 3 IdVdaiT LIBRARI ES^ SMITHSONIAN” INSTITUTION NOlinilXS

r- z r~ , Z r~ Z [3 Z

o H XV ° O sXfa. m X?22£oX 2

* CD

* fit
X xKx, > 11
vXr </XX zo \a_

—

rn x^voshv/ rn vs^' xci22>^ m ^ m x^osv^

co E co — co _ co

dshiiins S3iavyan libraries Smithsonian institution NoumiiSNi nvinoshiiins S3iavaan librari

CO Z CO z T CO z

>°*oX 2:

' Xn -J- 'ft. t/'XXX-, o Wism T /•?£•*■ o W

CO

^ o

>’r 5 ' ^ 2 \T' > _ z w

hsonian"-institution ^NoiiniusNi NvmosHims^SB i yvy a n libraries smithsonian institution Noiinius

— CO 5 \ CO = CO ^ Z ^ CO

^ Cul y<5vnTi}^ ^ UJ ///fa. 77, ZTooa^x cu

CO

)SHimS ZS3 I yvy a n^LIBRAR I ES^SMITHSONIAN^INSTITUTION^NOIlflillSNI^NVINOSHlIWS ^S3 IdVdail ^LIBRARI

z r- z i- z _

co m x^ncy -'W*' m x^ov^ u> m x^j^y' m

z co _ co £ — co X £ co

HSONIAN “INSTITUTION NOIlDlllSNI NVIN0SH1IWS SBIdVdan LIBRARIES SMITHSONIAN INSTITUTION NOIlfUllS
z , co z co z co z

oshiiws saiyvyan libraries Smithsonian institution NoumiiSNi nvinoshiiws saiyvyan librarii

— m — tn r— rn HI

/IITHSONIAN INSTITUTION NOIlfUUSNI NVINOSH1IIAIS S3iavaai3 LIBRARIES SMITHSONIAN INSTITUTION NOIlfll

m /n P" • (f) —r , «

° X /o|

CO V CO s

w ,.v > S > '■

/inoshiiws^ss lavyan^LiBRARi es^smithsonian ^institution NouniiisNi nvinoshihms^sb iavaan\iBRA

•-in — m — <o \ in —

< c 31) I

' 5 NWE55H/ “

/IITHSONIAN~INSTITUTION NOlinillSNrWlNOSHimS^SH I BVHa n"*LI BRAR i es smithsonian~institution~noudi

*" - - z r~ _ . ? f- _ ^ z

m in m W- in WtfgftX m x£cd£>" ^ w m

co ± to \ z — co £ to —

/inoshiiiais saiavaan libraries Smithsonian institution NouniiisNi nvinoshimis saiavaan librai

5 ■<;•♦•■ ^ z CO 2 CO 2

GO

-v' .*iV ?* «cT

^ ^ ^ X^UiiS2> _ _

1ITHS0NIAN_INSTITUTI0N NOimillSNI NVINOSHIUMS^SS I ava a Ll B RAR I ES^SMITHSONIAN^INSTITUTION^NOIlfU
-5. co — <n x co x co

£§n£&\ w co ^ ... co

VJ*/A

c

H

o xgc p«-^ _ xiuiiss^ Q O X»/v pc^x — O

inoshiuns saiavaan libraries^smithsonian~jinstitution Nouniusm^NviNosHiiiAis^sai ava a n‘Ju bra

» z r~ z r* 2 — — —

O O I __ _ 1

CD

;o

> Ip!

m X'ftosnjy £; m xcyosv^ i" m ^ XjvAst^x m xtv/yo^

— co £ co £ co x co

iithsonian institution NouniiisNi nvinoshiiws S3 lava an libraries Smithsonian institution Nouni

^ . 2 CO 2 co .2 •, CO z , co

3 #Hi § /#% 2 J» § %% 2 -

o x v^r

— c

CO

-L. O

t z , /

> ' s >’ 2 -"\w > \i«!S/ 5 5 •

CO 2 co 2 CO A> 2 w ■ ■•“■ z

/iNOSHims S3 lava an libraries Smithsonian institution NouniiisNi nvinoshiiws saiavaan libra

- co ~ co — co — -y. w —

c

H

O Xcvos^i^ “ O

IITHSONIAN^INSTITUTION NOIlfUUSNI NVIN0SH1IINS S3 I ava a n^LI B RAR I ES SMITHSONIAN INSTITUTION NOlini

~ cd 2 cd — . .-CD 2 AS#**

m ^ ^ x^Qiij-s^x m m

MNosHiiws S3 1 avaa n“u b rar i es Smithsonian “institution NouniusNrNViNosHiiws^ss i ava a n“u b rai

U} iZ. . (S) ~r (j) 2^ rrx

SfSj o x o UK

H
X
CO

— 2
S 2 ^ > '<

IITHSONIAN _ INSTITUTION NOimillSNI NVINOSHIIINS^SB laVaan^LfBRARI ES^SMITHSONIAN^ INSTITUTION ^NOIlfU

2 ff? ^ ^ 5 ^v. «> . . 5 <?. i

?7 O

' (£> (

ENTRAINMENT

OF

LARVAL OYSTERS

SPECIAL EDITION NO. 3 OF THE AMERICAN MALACOLOGICAL BULLETIN

SPECIAL EDITION NO. 3

OCTOBER 1986

CONTENTS

Entrainment of larval oysters by hydraulic cutterhead

dredges: an introduction. ROBERT S. PREZANT 1

An argument for retaining periods of non-dredging for the protection of oyster

resources in upper Chesapeake Bay. W. R. CARTER, III 5

Evaluating the need for dredging restrictions due to oyster larvae

entrainment. H. GLENN EARHART 11

The public oyster bottoms in Virginia: an overview of their size, location, and

productivity. DEXTER S. HAVEN and JAMES P. WHITCOMB 17

Expected seasonal presence of Crassostrea virginica (Gmelin) larval populations,

emphasizing Chesapeake Bay. VICTOR S. KENNEDY 25

Physicochemical alterations of the environment associated with hydraulic

cutterhead dredging. JOHN D. LUNZ and MARK W. LaSALLE 31

Prediction of flow fields near the suction of a cutterhead dredge.

E. C. McNAIR, JR. and GLYNN E. BANKS 37

Influence of suspended particles on biology of oyster larvae in estuaries.

MELBOURNE R. CARRIKER 41

Arctica islandica (Linne) larvae: active depth regulators or passive particles.

ROGER MANN 51

A review of some factors that limit oyster recruitment in Chesapeake Bay.

GEORGE R. ABBE .59

Entrainment of oyster larvae by hydraulic cutterhead dredging operations: workshop
conclusions and recommendations. MELBOURNE R. CARRIKER,

MARK W. LaSALLE, ROGER MANN and DONALD W. PRITCHARD

71

AMERICAN MALACOLOGICAL BULLETIN

BOARD OF EDITORS

EDITOR

ROBERT S. PREZANT

Department of Biological Sciences
University of Southern Mississippi
Hattiesburg, Mississippi 39406-5018

MELBOURNE R. CARRIKER

College of Marine Studies
University of Delaware
Lewes, Delaware 19958

ASSOCIATE EDITORS

ROBERT ROBERTSON

Department of Malacology
The Academy of Natural Sciences
Philadelphia, Pennsylvania 19103

GEORGE M. DAVIS

Department of Malacology
The Academy of Natural Sciences
Philadelphia, Pennsylvania 19103

W. D. RUSSELL-HUNTER

Department of Biology
Syracuse University
Syracuse, New York 13210

R. TUCKER ABBOTT
American Malacologists, Inc.
Melbourne, Florida, U.S.A.

JOHN A. ALLEN
Marine Biological Station
Millport, United Kingdom

JOHN M. ARNOLD
University of Hawaii
Honolulu, Hawaii, U.S.A.

JOSEPH C. BRITTON
Texas Christian University
Fort Worth, Texas, U.S.A.

JOHN B. BURCH
University of Michigan
Ann Arbor, Michigan, U.S.A.

EDWIN W. CAKE, JR.

Gulf Coast Research Laboratory
Ocean Springs, Mississippi, U.S.A.

PETER CALOW
University of Sheffield
Sheffield, United Kingdom

BOARD OF REVIEWERS

JOSEPH G. CARTER
University of North Carolina
Chapel Hill, North Carolina, U.S.A.

ARTHUR H. CLARKE
Ecosearch, Inc.

Portland, Texas, U.S.A.

CLEMENT L. COUNTS, III
University of Delaware, U.S.A.

Lewes, Delaware, U.S.A.

THOMAS DIETZ
Louisiana State University
Baton Rouge, Louisiana, U.S.A.

WILLIAM K. EMERSON
American Museum of Natural History
New York, New York, U.S.A.

DOROTHEA FRANZEN
Illinois Wesleyan University
Bloomington, Illinois, U.S.A.

VERA FRETTER
University of Reading
Berkshire, United Kingdom

ROGER HANLON
University of Texas
Galveston, Texas, U.S.A.

JOSEPH HELLER

Hebrew University of Jerusalem

Jerusalem, Israel

ROBERT E. HILLMAN
Battelle, New England
Duxbury, Massachusetts, U.S.A.

K. ELAINE HOAGLAND
Academy of Natural Sciences
Philadelphia, Pennsylvania, U.S.A.

RICHARD S. HOUBRICK
U.S. National Museum
Washington, D.C., U.S.A.

VICTOR S. KENNEDY
University of Maryland
Cambridge, Maryland, U.S.A.

ALAN J. KOHN
University of Washington
Seattle, Washington, U.S.A.

ISSN 0740-2783

LOUISE RUSSERT KRAEMER
University of Arkansas
Fayetteville, Arkansas, U.S.A.

JOHN N. KRAEUTER
Baltimore Gas and Electric
Baltimore, Maryland, U.S.A.

ALAN M. KUZIRIAN
NINCDS-NIH at the
Marine Biological Laboratory
Woods Hole, Massachusetts, U.S.A.

RICHARD A. LUTZ

Rutgers University

Pise at a way, New Jersey, U.S.A.

EMILE A. MALEK

Tulane University

New Orleans, Louisiana, U.S.A.

MICHAEL MAZURKIEWICZ
University of Southern Maine
Portland, Maine, U.S.A.

JAMES H. McLEAN

Los Angeles County Museum

Los Angeles, California, U.S.A.

ROBERT F. McMAHON
University of Texas
Arlington, Texas, U.S.A.

ROBERT W. MENZEL
Florida State University
Tallahassee, Florida, U.S.A.

ANDREW C. MILLER
Waterways Experiment Station
Vicksburg, Mississippi, U.S.A.

BRIAN MORTON
University of Hong Kong

Hong Kong

JAMES J. MURRAY, JR.
University of Virginia
Charlottesville, Virginia, U.S.A.

RICHARD NEVES
Virginia Polytechnic Institute
and State University
Blacksburg, Virginia, U.S.A.

WINSTON F. PONDER
Australian Museum
Sydney, Australia

CLYDE F. E. ROPER
U.S. National Museum
Washington, D.C., U.S.A.

NORMAN W. RUNHAM
University College of North Wales
Bangor, United Kingdom

AM ELI E SCHELTEMA

Woods Hole Oceanographic Institution

Woods Hole, Massachusetts, U.S.A.

ALAN SOLEM

Field Museum of Natural History
Chicago, Illinois, U.S.A.

DAVID H. STANSBERY
Ohio State University
Columbus, Ohio, U.S.A.

FRED G. THOMPSON
University of Florida
Gainesville, Florida, U.S.A.

THOMAS E. THOMPSON
University of Bristol
Bristol, United Kingdom

NORMITSU WAT ABE
University of South Carolina
Columbia, South Carolina, U.S.A.

KARL M. WILBUR
Duke University

Durham, North Carolina, U.S.A.

Cover. Crassostrea virginica Gmelin from Chesapeake Bay. From Galtsoff, P. S. 1964. The American Oyster. Fishery Bulletin of the Fish and
Wildlife Service 64: 1-480. Reprinted in memory of the contributions of Paul S. Galtsoff to oyster biology.

THE AMERICAN MALACOLOGICAL BULLETIN (formerly the Bulletin of the American Malacological Union) is the official journal publication
of the American Malacological Union.

AMER. MALAC. BULL. SPECIAL EDITION NO. 3

October 1988

ENTRAINMENT OF LARVAL OYSTERS BY HYDRAULIC CUTTERHEAD

DREDGES: AN INTRODUCTION

ROBERT S. PREZANT
DEPARTMENT OF BIOLOGICAL SCIENCES
UNIVERSITY OF SOUTHERN MISSISSIPPI
HATTIESBURG, MS 39400-5018, U.S.A.

ABSTRACT

Reasons for the decline in commercial oyster fisheries in Chesapeake Bay include pollution,
predation, sedimentation, disease, and harvest pressures. Recently, members of the oyster industry
suggested another source of oyster mortality, i.e., entrainment of larval oysters by hydraulic cutterhead
dredges used to deepen and widen channels in the Bay’s tributaries. Accordingly, the Army Corps
of Engineers, Baltimore District, sponsored a workshop to address this possible problem. Participants
of the workshop offered conflicting views on the impact of dredging operations: either dredging has
a negligible impact on larval oyster populations (a reflection of an already large natural larval mortal-
ity), or dredging activities push larval mortality over some critical threshold that reduces future spat
settlement and thus adult oyster populations. Information on oyster larval physiology and physiological
responses to various deleterious environmental factors is necessary before questions concerning the
impacts of dredging on the viability of commercial oyster populations can be addressed.

The productive oyster reefs of the coastal regions of
the northeastern United States produce over 50,000 thousand
pounds (five year average for 1 978-1 982) of the American oys-
ter Crassostrea virginica (Gmelin) per year (USDG, 1984).
While this may seem an impressive amount, it is less than
20% of the annual harvest of the late 1800’s (Lyles, 1969).
There are many reasons for this decline in productivity (Mac-
Kenzie, 1970). Galtsoff (1964) discussed several factors that
regulate oyster populations including sedimentation (i.e. burial
and suffocation of spat), “disease”, predators (including gas-
tropods, asteroids, turbellarians, fish, birds, man), commen-
sals and competitors (including boring sponges, boring clams,
mud worms, oyster crabs, spirochaetes, perforating algae,
“fouling organisms”), and pollution. More recently, Seliger
et at. (1985) increased the list to include anoxia caused by
an increase in land runoff and sewage effluent input.

Larval mortality is another important factor in the
regulation of molluscan communities. In fact, Thorson (1950)
noted that for organisms with free swimming larval stages,
larval mortality can be one of the prime regulators of the
population. There are many causes of larval mortality among
marine and estuarine molluscs. Vecchione (1986) briefly
outlined some of these, including starvation (Beyer, 1980;
Anger ef a/., 1981), predation (Mileikovsky, 1974; Steinberg
and Kennedy, 1979), loss of larvae due to transport out of
viable environmental regimes (Norcross and Shaw, 1984),
and pollution (Roosenburg, ef a/., 1980; Wright et at., 1983).
These impacts on larval populations will have obvious effects

on oyster fisheries. The series of papers that follow in this
publication are among those presented at a workshop, spon-
sored by the Baltimore District of the Army Corps of
Engineers, designed to help understand another potential
strain on oyster populations: lethal entrainment of oyster lar-
vae by hydraulic cutterhead dredges. The workshop on En-
trainment of Larval Oysters by Cutterhead Dredges in
Chesapeake Bay, held at the University of Delaware, College
of Marine Studies in August 1985, summarized knowledge
on oyster larvae and the possible impact that entrainment
could have on their population viability. The workshop brought
together authorities on oyster biology, oyster fisheries and
dredging operations. The main question asked was: do
hydraulic cutterhead dredges lethally entrain large numbers
of larval oysters and if so to what extent will this reduce oyster
production in Chesapeake Bay? Individuals involved with
dredging operations suggest that dredging does not
significantly deter the already stressed oyster populations of
Chesapeake Bay and are requesting wider time frames for
their operations.

One of the goals of the workshop was to devise a
model that would allow predictions, based on present
knowledge, of potential dredging impacts. Another goal, and
one no less important, was to ask what additional informa-
tion is needed to adequately understand the interaction of
necessary dredging activities and oyster reef productivity in
Chesapeake Bay?

Lunz (1985) suggested that there would be no signifi-

American Malacological Bulletin, Special Edition No. 3(1 986): 1-4

1

2

LARVAL ENTRAINMENT

cant impact on oyster production in Chesapeake Bay caused
by dredge induced entrainment. He believes that population
levels of oysters in Chesapeake Bay are regulated by high
“natural” mortality of larvae and spat. Based on discussions
with oyster biologists, Lunz states two reasons for the possibly
insignificant influence that hydraulic cutterhead dredging has
on oyster fisheries. First, the dredge entrains only a small
volume of water relative to the volume of water flowing past
the dredge that might contain oyster larvae. Second, natural
mortality of oyster larvae (including fertilized egg through spat)
is 99.999967%, and dredge impact beyond this would be in-
significant. The Maryland Department of Natural Resources
disagrees (see Carter, 1986, this volume) and suggests that
dredging operations negatively influence the viability of oyster
populations through entrainment.

Behavior, morphology, and development of ostreid
bivalves have been carefully examined (see for example
Davis, 1958; Ranson, 1960; Ockelmann, 1962; Galtsoff, 1964;
Loosanoff et at., 1966; Pascual, 1971, 1972; Waller, 1981).
Ockelmann (1962) examined the development of several
marine and estuarine bivalves and discussed their distribu-
tional patterns along the Atlantic coast of Europe. Included
in his discussion is information on the lecithotrophic develop-
ment of Ostrea chilensis Philippi. He notes that, as is com-
mon with many ostreid species, O. chilensis has a short lar-
val stage in the plankton, enhancing its chances of remain-
ing in the appropriate environmental regime for continued on-
togenetic development. The behavior of oyster larvae is
thought to be at the heart of their ability to remain in an en-
vironmental regime conducive to their post-settlement lives.
Galtsoff (1964) suggested that during their two to three weeks
in the plankton, the veliger larvae of Crassostrea virginica are
essentially at the mercy of surrounding water movements and
are thus passively carried by currents to their final distribu-
tion. T. Nelson (1952) and M. R. Carriker (1951) elaborated
the ideas of Julius Nelson (Galtsoff, 1964) in the belief that
oyster larvae migrate vertically, taking advantage of tidal
cycles to remain within an estuary. Oyster larvae appear to
migrate upward during incoming tides and downward dur-
ing ebb tides, thus aiding in their retention in the estuary.
Evidence presented by Haskin (1964), Hidu and Haskin (1971)
and Newkirk (1978) indicates planktonic larvae of C. virginica
perceive salinity gradients and swim towards lower salinity
surface waters or sink into denser, higher salinity bottom
waters. The exact mechanisms involved or the existence of
this behavior remains speculative.

While the main emphasis of this workshop was entrain-
ment of larval oysters by hydraulic cutterhead dredges, there
are other potentially serious impacts caused by dredging ac-
tivities. Some of these are discussed in papers that follow.
One of particular import could be siltation (see Carriker, 1986,
this volume). Lunz et at. (1984) suggest that “Although
perhaps more sensitive than juvenile and adult stages, eggs
and larvae of shellfish species inhabiting turbid estuaries and
coastal waters can be expected to be adapted to and highly
tolerant of naturally occurring elevated suspended sediment
concentrations (eg. concentrations generated during storm
events and seasonal flooding conditions) for reasonable

durations of time.” Rapid desposition of suspended
sediments, however, can be detrimental to oyster com-
munities in particular habitats. Poor water circulation, caused
by construction of a continuous levy in Calcasieu Ship Chan-
nel in Louisiana, U.S.A., allowed excess siltation in Calcasieu
Lake that destroyed the annual one million pound oyster
harvest (Louisiana Sea Grant College Program, 1985). Work-
ing in conjunction with the Louisiana Department of Wildlife
and Fisheries, the Army Corps of Engineers developed a plan
that restricted the deposition of dredge spoil along the levy
thus allowing restoration of adequate water flow into the
oyster fisheries area. The substantial oyster harvest has since
been restored. Priest (1981) quantitifed impacts of suspend-
ed sediments on oyster larvae. Lunz et at. (1984) agree that
if detrimental concentrations are created by dredging opera-
tions, they should be modified or curtailed.

Galtsoff (1 964) suggested that there is no evidence that
larvae of C. virginica are selective in their ability to find a
suitable place to set . . provided the surface is not covered
with a slimy film, detritus, or soft mud.” It is possible that man
induced perturbations influence natural settlement of larvae
by obscuring settlement cues. Invertebrate larvae reach
metamorphic competence only after a critical ontogenetic
stage. Frequently these larvae retain metamorphic com-
petence and hold off metamorphosis until arriving at an ap-
propriate settlement site (Kempf, 1981). Sometimes larvae
become less discriminating with time and can reach a stage
of spontaneous metamorphosis (Pechenik, 1980; Kempf,
1981). If oyster larvae are not appropriately cued because
the stimulus is obscured, they can metamorphose spon-
taneously in an environment that is not conducive to survival.

The impact of a mechanical “predator” (i.e. the cut-
terhead dredge suction end) will also be regulated by the
“volume” of oyster larvae exposed to it. To a great extent
this relies on whether oyster larvae are homogeneously
dispersed in the water column or, as is more likely, travel or
are carried in swarms or cloud-like aggregates. If larvae are
dispersed in clouds, at a particular time it is unlikely that they
would be subject to entrainment by the dredge; on the other
hand a single cloud of larvae could be entrained en toto.

There is only a small data base on the physiology and
physiological responses of larval oysters to various impacts,
a matter of some concern to decision makers when dealing
with commercial fisheries and larval activities as influenced
by man. More information is required before we can answer
questions about the impact of environmental perturbations
on a viable commercial oyster population. At present, no in-
formation is available concerning interaction of larval oysters
and hydraulic cutterhead dredges, nor has the potential been
examined for any substantial impact on the viability of future
oyster populations based on entrainment by these dredge
operations. Additionally, sweeping generalizations cannot be
made concerning larval movement in particular bodies of
water without a firm understanding of circulation patterns in
that area. Thus, if we could model larval behavioral interac-
tions in estuarine systems impacted by dredging, we would
have to take careful note of particular circulatory patterns in
addition to causes of larval mortality other than entrainment,

PREZANT: LARVAL OYSTER ENTRAINMENT

3

and the viability of commercial oyster fisheries in that area
in the absence of dredging. That is, any model must take in-
to account many features that regulate oyster populations and
be able to predict the effect on those populations by the ad-
dition or subtraction of one or any variable used in that
model.

One feature that can be manipulated to help control
potential impacts on oyster fisheries is the time during which
dredging is allowed. Periods of allowable dredging, termed
“windows”, are frequently established to prevent dredging
during critical periods in the life history of particular species.
Seasonal restrictions on dredging operations to prevent en-
trainment of migrating fish have been instituted in several
areas (see Lunz et at., 1984). Corps districts that have
previously restricted dredging operations to protect
“shellfish” larvae have done so to prevent the adverse ef-
fect of increased suspended sediments and sedimentation
on bivalve eggs, larvae and spat (Norfolk District) and an in-
crease in pathogenic bacteria that can debilitate hard clam,
Mercenaria mercenaria Linne, larvae (New York District) (see
Lunz et a!., 1984).

The papers presented in this volume cover a variety
of topics dealing with questions of larval entrainment, con-
centrating on Chesapeake Bay. To outline the framework of
the workshop, papers by Earhart (1986) and Carter (1986)
set the tone of the debate; that is, is hydraulic dredging a
serious threat to oyster fisheries in Chesapeake Bay and
based on the answer to this, should we consider retention
of stringent dredging windows or can these windows be
modified? Several papers included here deal with oyster beds
of Chesapeake Bay, reproductive and larval biology of
oysters, and larval transport (see Haven and Whitcomb, 1986;
Kennedy, 1986; Mann, 1986). McNair and Banks (1986)
discuss the flow fields surrounding the action end of hydraulic
cutterhead dredges while Lunz and LaSalle (1 986) outline the
physicochemical impacts occurring at the dredge head. A
model devised during the workshop that deals with the pre-
sent situation is discussed in a summary paper by Carriker
et al. (1 986). Many questions are raised by authors that em-
phasize the need for additional research before we can truly
understand the possible synergistic effects that man induced
perturbations can have on estuarine communities. There re-
main two views, even following the workshop; 1) the impact
of hydraulic cutterhead dredging is neglible in the face of the
natural causes of heavy oyster larvae mortality and dredg-
ing windows can be expanded without harm or 2) the addi-
tional impact of expanded windows from hydraulic cutterhead
dredging activities can increase larval mortality to a degree
that would seriously effect the viability of future oyster
fisheries. The biological “truth” most probably lies
somewhere in between these two extremes and only careful
and well controlled research will reveal a definitive answer.
Papers in this volume give an indication of the level of our
current knowledge and set the stage for future research.

ACKNOWLEDGMENTS

Support for the workshop and this publication has been sup-

plied by the Baltimore District of the Army Corps of Engineers,
Maryland and the Waterways Experiment Station, Vicksburg,
Mississippi. John Lunz and Glenn Earhart were instrumental in
generating the support for this effort. The success of the workshop
was directly related to the dedication of the participants and all
deserve high commendations and thanks for their efforts. Mark
LaSalle was a key coordinator of this undertaking and his continued
enthusiasm helped tremendously. Thanks also to Kashane Chalerm-
wat and Antonieto Tan Tiu for much needed help during the
preparatory stages of the workshop. Melbourne R. Carriker kindly
acted as editor and reviewer for this manuscript. Two additional
anonymous reviewers helped make this a more cohesive manuscript.
The workshop was held at the Virden Center, College of Marine
Studies, University of Delaware, Lewes, Delaware and the expert
help of the Virden Center staff assured a successful meeting.

LITERATURE CITED

Anger, K., R. R. Dawirs, V. Anger and J. D. Costlow. 1981 . Effects
of early starvation periods on zoeal development of brachyuran
crabs. Biological Bulletin 161:199-212.

Beyer, J. E. 1980. Feeding success of clupeoid fish larvae and
stochastic thinking. Dana 1:65-91.

Carriker, M. R. 1951 . Ecological observations on the distribution of
oyster larvae in New Jersey estuaries. Ecological Monographs
21:19-38.

Carriker, M. R. 1986. Influence of suspended particles on biology
of oyster larvae in estuaries. Entrainment of Larval Oysters,
American Malacological Bulletin Special Edition No. 3:41-49.
Carriker, M. R., R. Mann, M. W. LaSalle and D. W. Pritchard. 1986.
Entrainment of oyster larvae by hydraulic cutterhead dredg-
ing operations: workshop conclusions and recommendations.
Entrainment of Larval Oysters, American Malacological Bulletin
Special Edition No. 3:71-74.

Carter, W. R. III. 1986. An argument for retaining periods of non-
dredging for the protection of oyster resources in upper
Chesapeake Bay. Entrainment of Larval^Oysters, American
Malacological Bulletin Special Edition No. 3:5-10.

Davis, H. C. 1958. Survival and growth of clam and oyster larvae
at different salinities. Biological Bulletin 114:296-307.
Earhart, H. G. 1986. Evaluating the need for dredging restrictions
due to oyster larvae entrainment. Entrainment of Larval
Oysters, American Malacological Bulletin Special Edition
No. 3:11-16.

Galtsoff, P. S. 1964. The American oyster Crassostrea virginica
Gmelin. Fishery Bulletin 64:1-480.

Haskin, H. H. 1964. The distribution of oyster larvae. Occasional
Publication of Proceedings of Symposium on Experimental
Marine Ecology (University of Rhode Island) 2:76-80.
Haven, D. S. and J. P. Whitcomb. 1986. The public oyster bottoms
in Virginia: an overview of their size, location and productiv-
ity. Entrainment of Larval Oysters, American Malacological
Bulletin Special Edition No. 3:17-23.

Hidu, H. and H. H. Haskin. 1971. Settling of the American oyster
related to environmental factors and larval behavior. Pro-
ceedings of the National Shellfisheries Association 61 :35-50.
Kempf, S. C. 1 981 . Long-lived larvae of the gastropod Aplysia juliana:
do they disperse and metamorphose or just slowly fade away?
Marine Ecology - Progress Series 6:61-65.

Kennedy, V. S. 1986. Expected seasonal presence of Crassostrea
virginica (Gmelin) larval populations, emphasizing Chesapeake
Bay. Entrainment of Larval Oysters, American Malacological
Bulletin Special Edition No. 3:25-29.

4

LARVAL ENTRAINMENT

Loosanoff, V. L., H. C. Davis and P. E. Chanley. 1966. Dimensions
and shapes of larvae of some marine bivalve mollusks.
Malacologia 4:351-435.

Louisiana Sea Grant College Program. 1985. The persistent oyster.
Aquanotes 14(2): 1-4.

Lunz, J. D. 1985. An analysis of available information concerning
the entrainment of oyster larvae during hydraulic cutterhead
dredging operations with commentary on the reasonableness
of seasonally restrictive dredging windows. Prepared in
response to a request by the U. S. Army Engineer District,
Baltimore for assistance through the Dredging Operations
Technical Support (DOTS) Program. Waterways Experiment
Station, Vicksburg, Mississippi. 10 pp.

Lunz, J. D., D. G. Clarke and T. J. Fredette. 1984. Seasonal restric-
tions on dredging operations. A critical review of the arguments
for seasonal restrictions with suggested criteria and a method
to determine the need for seasonal restrictions on federal and
permit dredging projects. Prepared in response to a request
for assistance made by the Regulatory Branch, Operations
Division, New York District (NANOP-E) through the Dredging
Operations Technical Support (DOTS) Program. 112 pp.

Lunz, J. D. and M. W. LaSalle. 1986 Physicochemical alterations
of the environment associated with hydraulic cutterhead
dredging. Entrainment of Larval Oysters, American Mala-
cological Bulletin Special Edition No. 3:31-36.

Lyles, C. H. 1965. Fishery statistics of the United States. Statistics
Digest 59:1-756.

MacKenzie, C. L., Jr. 1970. Causes of oyster spat mortality, condi-
tions of oyster settling beds, and recommendations for oyster
bed management. Proceedings of the National Shellfisheries
Association 60:59-67.

Mann, R. 1986. Arctica islandica (Linne) larvae: active depth
regulators or passive particles. Entrainment of Larval Oysters,
American Malacological Bulletin Special Edition No. 3:51-57.

McNair, E. C. and G. E. Banks. 1986. Prediction of flow fields near
the suction of a cutterhead dredge. Entrainment of Larval
Oysters, American Malacological Bulletin Special Edition
No. 3:37-40.

Mileikovsky, S. A. 1974. On predation of pelagic larvae and early
juveniles of marine bottom invertebrates and their passing
alive through their predators. Marine Biology 26:303-311.

Nelson, T. C. 1955. Observations on the behavior and distribution
of oyster larvae. Proceedings of the National Shellfisheries
Association 45:27-28.

Newkirk, G. 1978. Interaction of genotype and salinity in larvae of
the oyster Crassostrea virginica. Marine Biology 48:227-234.

Norcross, B. L. and R. F. Shaw. 1984. Oceanic and estuarine
transport of fish eggs and larvae: a review. Transactions of

the American Fisheries Society 113:153-165.

Ockelmann, K. W. 1962. Developmental types in marine bivalves
and their distribution along the Atlantic coast of Europe. Pro-
ceedings of the First European Malacological Congress pp.
25-35.

Pascual, E. 1971 . Morfologia de la Charnela larvaria de Crassostrea
angulata (Lmk.) en diferentes fases de su desarollo. Investiga-
cion Pesquera 35:549-563.

Pascual, E. 1972. Estudio des las conchas larvarias de Ostrea sten-
tina, Payr. y Ostrea edulis L. Investigacion Pesquera
36:297-310.

Pechenik, J. A. 1980. Growth and energy balance during the larval
lives of three prosobranch gastropods. Journal of Experimen-
tal Marine Biology and Ecology 44:1-28.

Priest, W. I. 1 981 . The effects of dredging impacts on water quality
and estuarine organisms: a literature review. Virginia Institute
of Marine Science Special Report on Applied Marine Science
and Ocean Engineering 247:240-266.

Ranson, G. 1960. Les prodissoconques (coquilles larvaires) des
Ostreides vivants. Bulletin de TInstitut Oceanographie (Monaco)
No. 1183:1-41.

Roosenburg, W. H., J. C. Rhoderick, J. M. Block, V. S. Kennedy,
S. R. Gullans, S. M. Vreenegoor, A. Rosenkranz and C. Col-
lette. 1980. Effects of chlorine-produced oxidants on survival
of larvae of the oyster Crassostrea virginica. Marine Biology
- Progress Series 3:93-96.

Seliger, H. H., J. A. Boggs and W. H. Biggley. 1985. Catastrophic
anoxia in the Chesapeake Bay (U.S.A.) in 1984. Science
228:70-73.

Steinberg, P. D. and V. S. Kennedy. 1979. Predation upon
Crassostrea virginica (Gmelin) larvae by two invertebrate
species common to Chesapeake Bay oyster bars. Veliger
22:78-84.

Thorson, G. 1950. Reproductive and larval ecology of marine bot-
tom invertebrates. Biological Reviews 25:1-45.

U. S. Department of Commerce, NOAA, NMFS. 1984. Fisheries of
the United States, 1983. Current Fishery Statistics No. 8320.

122 pp.

Vecchione, M. 1986. The international symposium on the ecology
of larval molluscs: introduction and summary. American
Malacological Bulletin 4:45-48.

Waller, T. R. 1981. Functional morphology and development of veliger
larvae of the European oyster, Ostrea edulis Linne. Smith-
sonian Contributions to Zoology No. 328: 70 pp.

Wright, D. A., V. S. Kennedy, W. H. Roosenburg, M. Castagna and
J. A. Mihursky. 1983. Temperature tolerance of embryos and
larvae of five bivalve species under simulated power plant en-
trainment conditions: a synthesis. Marine Biology 271-278.

AN ARGUMENT FOR RETAINING PERIODS OF NON-DREDGING FOR THE
PROTECTION OF OYSTER RESOURCES IN UPPER CHESAPEAKE BAY

W. R. CARTER, III

MARYLAND DEPARTMENT OF NATURAL RESOURCES
TAWES STATE OFFICE BUILDING, C-2
ANNAPOLIS, MARYLAND 21401, U.S.A.

ABSTRACT

A sample of a hypothetical, stably reproducing oyster population is postulated, with given rates
of natural mortality in the larval and post-setting, pre-seed stages. The effects of additional larval mortality
caused by entrainment during a hydraulic dredging operation are shown to be potentially significant
in areas where dredging takes place near oyster setting grounds. Survival rates to seed stage are
modelled as being reduced by 2 to 19%. Larval survival rates are modelled as being reduced by 12
to 51%.

This paper addresses the question “Can dredging-
induced mortalities of oyster larvae be significant to the abun-
dance of adult American oysters Crassostrea virginica
(Gmelin)?” The conclusions are based on numerous assump-
tions due to the absence of data and variability that occurs
in the natural environment. They are necessarily sensitive
to the assumptions utilized. Nevertheless, I believe the values
assumed for rates in this paper are within realistic ranges.

The present approach involves identification of some
parameters that bear on the question and then postulation
of a representative sample of a hypothetical, stably reproduc-
ing oyster population. This utilizes a number of assumptions
concerning the age composition, sex ratio, and fecundity of
the population, natural mortality races of different sub-adult
life stages, and commercial harvest rates of adults.

In the late larval stages, an additional source of mor-
tality is introduced, that due to dredge entrainment. I treat
this additional source in computations as though it were
essentially fishing mortality. The treatment utilizes informa-
tion on dredging performance concerning water volume en-
trained, assumptions of larval density in the water column,
and assumptions of natural mortality rates in order to compute
altered total mortality rates. The resultant rates are then used
with numerical examples to show their effects upon previously
postulated hypothetical oyster populations.

Finally, a brief review of the current status of oyster
reproduction in Maryland is given, showing that my assump-
tions of stable populations and natural mortality rates
associated with them are conservative.

A HYPOTHETICAL, STABLY REPRODUCING
OYSTER POPULATION

Lunz (unpubl., March 1985) cited information to the

effect that larval mortality rates prior to spat set approximated
0.99999967, and that mortality from spat set to seed stage
could be an additional 60 to 90%. Webster and Shaw (1 968)
found post setting survival from June to September ranged
from 1 to 27%.

Webster and Shaw (1968) also found that higher set-
ting densities were associated with lower post-setting survival
rates, and that survival on bottom cultch averaged about Vz
that on suspended cultch. Their 1 to 27% survival rates were
computed from suspended cultch. Krantz (Maryland Dept,
of Natural Resources, 29 July 1985) suggested that post-
setting, pre-seed stage mortality could range as high as 99%.
Haskin and Tweed (1976) found setting-to-late-fall mortalities
ranging from 61.3 to 91.7% in Delaware Bay.

Krantz (Maryland Dept, of Natural Resources, 29 July
1985) stated that, in Maryland, oyster harvesting pressure
essentially completely removes a year class by the time it
is seven years old. Oysters become legally harvestable in
Maryland at three inches (76 mm) height, when they are about
three years old.

If oysters from age three are subject to a fishing
pressure that completely harvests a year class in the ensuing
four years, and if that pressure is constant and natural mor-
tality is neglected for these age groups three-seven, and if,
as Krantz suggested, 500 spat at the beginning of their first
winter will produce 375 three-year-old harvestables, a
hypothetical, stable population will have approximately the
following age composition:1

^he postulated age structure is derived from the assumptions
above, such that 500 spat at the beginning or their first winter die
off to leave 375 survivors at three years old, thus having an annual
natural mortality rate (“A’’) of 9.14%. Similarly, if 375 3 year olds
are, say 99% harvested (i.e., essentially to zero remaining) over a
four year period, their annual mortality due to fishing is 67.86%.

American Malacological Bulletin, Special Edition No. 3(1 986):5-1 0

5

6

LARVAL ENTRAINMENT

Age

Number

0 = spat

500

1 = seed

454

2

413

3

375

4

121

5

39

6

13

7

4

This age structure is the postulated representative
sample used in this paper.

Galtsoff (1964:299) states that the gonads of mature
oysters comprise from 32.8 to 33.4% of the volume of the
oyster’s body. He also gives figures (1964:313) for numbers
of eggs discharged in single spawnings for several oysters,
these individuals being 91 to 132 mm in height. I assumed
individuals of this height range to be 4 years old. The average
of Galtsoff ’s estimated numbers of eggs is 57.6 x 106. He
gives a figure of 1 1 5 x 1 06 eggs as discharged from a gonad
3.8 cm3 in volume.

Proportioning 115 x 106 eggs and 3.8 cm3 gonad
volume to 57.6 x 10® eggs gives a computed gonad volume
(for this mean egg number) of 1 .9 cm3. Dividing that volume
by the mean number of eggs (57.6 x 1 0®) gives an egg’s mean
volume as 33,043 /im3, close to Galtsoff’s estimated volume
for a single egg of 33,510 ^m3.

An allowance (according to Galtsoff) of 25% additional
volume for spaces between eggs results in a total gonad
volume of 2.4 cm3 for the oyster producing 57.6 x 10® eggs
per spawning. If this approximates 33.1 % of the body volume,
body volume for the oyster is 7.25 cm3.

I assumed that this volume applied to a four year old
oyster and that all other age groups one to seven were pro-
portional in volume as they were in body mass. Postulated
typical body masses for oysters of given age groups were

Table 1 . Age and sex composition, fecundity, and number of larvae
computed for representive sample of hypothetical stable oyster
population.

Age

Number

Eggs per Number of
Males Females female Larvae
x 107 x 109

settled

spat

6667

spat at

winter

0

500

“seed"

1

454

227

227

0.9

2.0

2

413

206

207

2.7

5.6

3

375

187

188

4.5

8.5

4

121

60

61

5.8

3.5

5

39

13

26

7.1

1.8

6

13

3

10

8.2

0.8

7

4

1

3

9.2

0.3

Sum

22.5

obtained from Krantz (Maryland Dept, of Natural Resources,
29 July 1985).

The postulated body masses were as follows:

Age Mass (g)

1 2.4

2 7.0

3 11.5

4 15.0

5 18.5

6 21.2

7 23.8

With these proportions I computed gonad volume and
then, deleting for the inter-egg space, the number of eggs
per female, as follows:

Gonad volume Number Eggs
ixvr\3 per female

Age x 1011 x 107

1 3.87 0.9

2 11.16 2.7

3 18.74 4.5

4 23.99 5.8

5 29.53 7.1

6 33.83 8.2

7 38.16 9.2

Galtsoff reported that the male to female ratio was
essentially 1:1 for the first four age groups. Kennedy (1 983),
investigating oyster bars in Chesapeake Bay, found that
oysters in age group five were 2:1 female to male; in age
group six, ratios were approximately 3:1 female to male, and
in age group seven, the female to male ratio was approximate-
ly 4.8:1.

The foregoing information on age composition, sex
ratio, and fecundity, shown on Table 1, is used a derive a
hypothetical abundance of larvae (assuming 100% fertiliza-
tion; a conservative assumption for the purpose of model-
ling effects of dredge-induced mortality) for the postulated
representative sample of the oyster population.

It should be emphasized here that Table 1 deals only
with a representative sample of the population. Later in the
paper, for the purpose of modelling effects of dredging, I ex-
pand this sample to represent the number of larvae produced
by a one hectare oyster bar having the same age, sex, and
fecundity characteristics.

NATURAL MORTALITY OF OYSTER LARVAE

The initial number of oyster larvae begins to die off
from natural causes immediately. Lunz’s (1985) cited rate of
survival is approximately 3.3 larvae surviving to spatfall per
10 million spawned. I assume that, for a 21 day larval life,
the rate of mortality is constant. That is, the same proportion
of the previous day’s surviving larvae die off each day. This
appears to be consistent with Korringa’s (1941) conclusion
that the longer the duration of larval life, the lower the number
reaching settlement. Thus, in warmer conditions, other fac-
tors being equal, larvae would mature sooner and a larger
initial spatfall would result. I acknowledge that some factors

CARTER: ARGUMENT FOR NON-DREDGING PERIODS

7

may intrinsically be unequal under different temperature
regimes, such as the mortality induced by varying bacterial
populations. However, highly variable complicating functions
could be introduced at any point in the life cycle of a
hypothetical population and would have no additional clari-
fying or instructive value.

In fisheries population dynamics, the instantaneous
rate of total mortality is referred to as “Z” (Ricker, 1 975). This
is defined as the negative log of the survival rate for the period
under consideration, in this case, 21 days. Stated
mathematically, Z = -loge(1 -A), where A is the fraction of
the original number that dies during the period under con-
sideration. (1 -A) = S, or the survival rate.

Thus, if 3.3 x 10'7 is the rate of survival for 21 days,
Z is computed as its negative loge, or loge 3.3 x 10'7 =
-14.9242, and Z = 14.9242. The daily rate of instantaneous
mortality is 14.9242 divided by 21, or 0.7101.

Given the daily rate, the number or the proportion of
the initial number of larvae remaining on any day of larval
life is computable. In standard fisheries usage, Z is comprised
of M, the instantaneous rate of natural mortality, and F, the
instantaneous rate of fishing mortality. Since 14.9242 = Z
is not currently refinable so as to discriminate anthropogenic
factors, it is equivalent to M. It will be so considered for the
remainder of this discussion. In the issue under considera-
tion, the effects of dredge-related mortality, death of larvae
through entrainment is analogous to fishing mortality, since
it is a source of death added to natural mortality. It is treated
as fishing mortality in the remainder of this discussion.

F, the larval mortality due to dredging is not precisely
additive to M. This is because after the dredging begins to
kill larvae, some of those entrained would have died by natural
causes, and some of those that would have died by natural
causes will instead be killed by the dredge. This interaction
requires that there be an adjustment between the two sources
of mortality. The relationship makes use of the terms “con-
ditional rate of fishing mortality, ‘m”\ and “conditional rate
of natural mortality, ‘n”\ The terms are related by the function
m + n - (mn) = A,

where A is the fractional or decimal value of mortality occur-
ring during the period under consideration (Ricker, 1975). The
term n is defined as n = 1 - e'^. The term m is defined as
m = 1 -e‘F.

Whereas n can be stated at this point as n = 1 - e’
■7101 _ i . .4gi3 = 0.5087, m requires further derivation.
This is covered in the following section.

PARAMETERS OF A DREDGING OPERATION

The amount of mortality caused by the dredging opera-
tion depends on how many larvae are entrained, apart from
any questions dealing with siltation. This in turn depends on
where the larvae are in relation to the water entrained, the
density of larvae in the water column, the volume of water
entrained per unit time, and how long the dredging goes on
while larvae are available to be entrained.

The volume of water entrained is a function of the

diameter of the dredge's pipeline, the velocity of water
through the line, and the length of time the dredge is active.
According to Reikenis (Century Engineering, pers. comm. 5
August 1985), typical pipeline dredges working in
Chesapeake Bay have diameters equal to or greater than 20.3
cm, many being 40.6 to 76.2 cm, with a few used for very
large jobs (several million cubic yards) being as large as 91 .4
cm. He stated that in-pipe velocities vary around 4.9 m/s,
which is sufficient to keep sand in suspension. Snow (Great
Lakes Dredge and Dock, pers. comm. 9 August 1985) stated
that in-pipe velocities varied between 4.6 and 6.1 m/s, being
greater for fine-grained materials and slower for coarse
materials. Snow noted that as the proportion of solids in the
slurry increased, slurry velocities decreased. He further noted
that proportions of slurry solids varied from 8-12% up to
25-28%, with the higher proportions being achieved in loose,
fine-grained substrata and the lower in tight clays or heavy
sand and gravel to cobble bottoms. He stated that a good
average of actual pumping time in the course of a day would
be 18 hours.

For this discussion, I assume a dredge pipeline of 40.6
cm diameter (cross-sectional area = 0.1297 m2), with an in-
pipe velocity of 4.8768 m/s, and a slurry composition of 28%
solid material. I assume an 18 hour pumping day. Larger
diameters, greater velocities, longer working hours, and lower
percentage solids would increase the volume of water
entrained.

A second major consideration is the density of larvae
in the water column. Pritchard (cited in Galtsoff, 1964)
estimated that spat sets sufficient to produce a commercial-
ly harvestable oyster crop could occur from a larval density
of 1 per 100 liters (= 10/m3). However, Galtsoff (1964:369)
notes that many observers have found that newly attached
spat “far outnumber” the free-swimming larvae taken in
plankton samples. He discussed the necessity for improved
methods of plankton sampling, assuming the desirability of
knowing planktonic stage densities. Korringa (1 941 : 1 09-1 1 0)
reported densities of larvae of Ostrea edulis Linne ranging
from 1980 to 2637/m3. Kern (NMFS Laboratory, Oxford,
Maryland) stated maximum personal observations in the
1960’s of 15/litre (= 15,000/m3). He stated that he thought
15 - 16/litre would be an average of high end-of-range obser-
vations, and that 5/litre could be a representative value for
the mode of observations during spawning seasons. He had
himself observed concentrations including straight hinge to
eyed stages as high as 12/litre.

No discernable relationships have been demonstrated
relating larval density to spat set and earlier programs for
sampling planktonic stage abundance have been discontinued
(Lunz, unpub., March 1985). Kern (pers. comm. 9 August
1 985) was involved in such a program and stated that better
indications of the strength of sets and the resultant year
classes were obtainable with less cost and effort by spat col-
lection techniques, although his samples of plankton show-
ing periodic increases of abundance tended to be related in
time to the appearance of spat on collectors. Intuitively, it ap-
pears that the abundance of late stage larvae must be related
to the abundance of early stage settled spat. Difficulties in

8

LARVAL ENTRAINMENT

showing regular relationships are likely to be related to dif-
ficulties of sampling representatively. Kennedy and Breisch
[1981; citing Pritchard’s 1 953 proposal] note that larvae could
swarm and thus appear in various peaks of concentration over
time at one station. Galtsoff (1964) cited observations of
Nelson and of Carriker that appeared to indicate that larvae
were distributed in definite lanes up and downstream from
spawning grounds. He stated that in homogeneous salinity
conditions, the greatest number of larvae is found at the level
(in the water column) of highest current velocity.

With regard to both difficulties of representative sampl-
ing and the distribution of concentrations of older, near-to-
setting larvae, Kennedy and Breisch [1981; citing Carriker
(1951)] note that eyed larvae (the stage preceding setting)
were more abundant on or near the bottom during ebb tide
than at flood, and that more larvae were collected directly
on the bottom than off bottom during flood tide. Kennedy and
Breisch (1981) also cite Manning and Whaley (1954) as hav-
ing found older larvae to be more predominant in the lower
water column. Kunkle (1957) [cited in Kennedy and Breisch
(1981)] reported that late stage larvae tend to concentrate on
or near the bottom at slack tides, ebb tides, and in the late
stages of flood. Carriker (1961) summarized then current
views on larval oyster dispersal and noted a concensus that
fully developed pediveligers have a tendency to remain in
lower, more saline strata thus being carried upstream by
return flows of deeper, denser water. Channel areas are
deeper than surrounding flats; they are likely to be the areas
with stronger currents; they are likely to be the areas through
which more saline, bottom-hugging water strata intrude into
an area prior to their final dispersal.

I conclude, therefore, that for the last few days of the
larval stage, oysters are likely to be found in channel areas,
and that the channels closest to areas of setting are likely
to have large concentrations of larvae. I assume that the bot-
tom two meters of the water column is where late stage lar-
vae are concentrated during most of the 24-hour day.

Since dredging operations are frequently involved in
deepening channels, or when making new channels tend to
take advantage of the pre-existing deeper areas in order to
minimize the amount of material to be moved, I conclude that
aggregations of later-stage larvae can be vulnerable to entrain-
ment. For the purposes of the subsequent computations, I
assume that since the dredge intake is on the bottom, it will
tend to entrain the deeper strata preferentially. I utilize the
same two meter deep bottom water stratum containing lar-
vae as the most likely to be entrained. I assume, for the com-
putations, a value of 5,000 larvae/m3 in the water entrained.

Finally, although it does not mathematically affect the
computations dealing with the impact of entrainment, I reason
that the older larvae, being survivors from a period of intense
mortality, are those that are “more valuable”1 (relative to
earlier stage larvae) to the replenishment of the bars on which
they could set, since the older they get, the greater the chance
that they will survive to set.

Given the assumptions of dredging performance and
density of oyster larvae in the water column, I now derive the
value of “m”, the conditional rate of fishing mortality, as

follows:

1. Dredge with pipeline cross-sectional area 0.1297 m2 and

intake velocity 4.8768 m/s entrains 0.6325 m3/s. Work-
ing for 18 hours, such a dredge can entrain 40.987 m3,
equivalent to 2.0493 hectares, 2 meters deep.

2. At 28% solid slurry, volume of water entrained = 29,510

m3.

3. At larval density 5,000/m3, number of larvae entrained in

one day = 147,550,000, ca. 1.5 x 108.

4. In order to relate the real world volumes of water en-

trainable by a dredge to the number of larvae in near-
by water, it is convenient to expand the representative
sample’s larval production (discussed above) to
numbers that might derive from a highly productive,
stable one-hectare oyster bar. I assume a bar that could
support an intense annual harvest of 1,000
bushels/acre, or 2,471 , bushels/hectare. The relation-
ship to the “representative sample” is that the “sam-
ple” is able to support a one bushel annual harvest.
Thus, the expansion simply increases the
characteristics of the sample by an area factor, 2,471 .
The expansion allows computing larval production of
5.6 x 1013 larvae per spawning. In the case of no man-induced
additional mortality, these larvae survive at the rate of 3.3
per 10 million spawned, leaving after 21 days approximately
1.8 x 107 new spat for the one hectare bar.

Larvae surviving natural mortality through day 15 =
1.304 billion, ca. 1.3 x 1 09, from the original number (5.6 x
1013) produced by a one hectare bar with the population
characteristics of the sample of Table 1 . Then, dividing the
larvae entrainable per day (1 .5 x 1 08) by the larvae surviving
natural mortality through day 15, (1 .3 x 109), I obtained the
estimate for “m” as 1.1 x 1 0'1 , or 0.1131.

Given values for “m” and “n”, I now calculate a new
value for daily Z, for the condition of additional dredge-
induced mortality operating from the beginning of day 16, as
follows:

1. m + n- (mn) = A Where n = 0.5087

A = 0.5643 m = 0.1131

2. -loge (1 - A) = Z A = fraction dying

-loge (0.4357) = Z = 0.8307 during period

(one day)

MODELLED EFFECTS OF DREDGING
ON OYSTER BAR VIABILITY
Galtsoff (1964:313) stated that the majority of female
oysters in a laboratory situation could be induced to spawn
two or three times in a six week period. Webster and Shaw
(1 968) found setting in tributaries of the lower Choptank from
late June through late September in 1 961 and from early June
through early September, 1962. Shaw (1969) [citing Beaven

1 An arbitrary, but possibly useful way of quantifying “increased
value” of an age group to the succeeding life stage would be to in-
dex it according to its achieved Z; the greater the odds against an
organism reaching a given age, the higher would be its potential value
to the next life stage.

CARTER: ARGUMENT FOR NON-DREDGING PERIODS

9

(1955)] reported that setting in Maryland can begin in late
May and extend well into October, with a peak lasting about
two weeks sometime during late June to September. Ken-
nedy (1980) reported lower Choptank River setting from early
July through mid-September, 1977, from early June through
early August, 1978, and from late May through mid-
September, 1979.

In order to make the present projections more realistic,

I assume three spawnings by females. For computational
simplicity I assume each to be of the same magnitude. I fur-
ther assume the first spawning to take place at 22°C, with
a planktonic life of 21 days. The second takes place at 29°C,
with a planktonic life duration of 16 days. The third is at 26°C,
with planktonic life lasting 18 days.

For the one hectare bar, the first spawning, at the
postulated natural mortality rate (daily Z = 0.7101) produces
ca. 1 .7 x 1 07 spat. The second produces 5.8 x 1 08 spat, and
the third 1 .4 x 108 spat. A dredge is postulated to begin opera-
tions at the beginning of day 16 for any of the three spawn-
ings. As specified above, it entrains approximately 1 .5 x 108
larvae per day, giving, for day 16 and any remaining days
of planktonic life a daily Z value of 0.8307. The new effective
Z is now utilized to calculate the effects on survival rate in
each spawning case. For each case, the number surviving
natural mortality through day 15 was 1.3 x 1 09, for the one
hectare bar’s production.

For the first spawning, with six remaining days of
planktonic life, the computation of the effect of the dredging
is as follows:

N — n "Zt

1 °e where N0 = 1.3 x 109
daily Z = 0.8307
t1 = 6 days

N2i = Nt = 8.9 x 1 06, for an ultimate survival rate over all
21 days of 1 .6 x 1 0'7. Without dredging, survival would have
been conditioned by a daily Z of 0.7101, for a 21 day sur-
vival rate of 3.3 x 1 0“7. Survival rate was reduced by 51 .4%.

For the second spawning, with only one day of
planktonic life remaining (t’ = 1), N16 = Nt = 5.7 x 1 08, an
ultimate survival rate over all 16 days of 1 .0 x 10'5. Without
dredging, the survival rate under a daily Z of 0.7101 would
have been, for all 16 days, 1.15 x 10"5. Survival rate was
reduced by 11.9%.

For the third spawning, with three days of planktonic
life remaining, (f = 3), the number surviving at N-,e = 10.8
x 1 07, for an 1 8 day survival rate of 1 .9 x 1 0'6. Without dredg-
ing, survival rate over 1 8 days would have been 2.78 x 1 0"6.
Survival rate was reduced by 30.7%.

Since three spawnings are assumed necessary for
enough spat set to maintain population stability, and popula-
tion stability implies 454 oysters (per representative sample)
reaching seed stage (= age 1), it is implicit that there are
differential survival rates among the three broods in the post-
setting period. As mentioned above, Webster and Shaw
(1968) found post-setting survival rates of 1 to 27% for off-
bottom cultch settings, and about one-third the off bottom
survival rate for spat that had set on cultch on the bottom.
They also found that higher setting densities were associated

with lower post-setting survival rates.

To complete the evaluation of potential dredging ef-
fects on bar stability, I assumed post-setting survival rates
that would give 500 spat per representative sample at the
beginning of winter added to the representative sample, under
the no-dredging scenario. These rates, for the three spawn-
ings respectively, were 0.29%, 0.075%, and 0.54%. All are
quite low because of the assumptions of a fixed initial Z value
and stability of the hypothetical oyster population. They could
be realistic, given the scenario of setting on bottom cultch,
a protracted period between setting and winter, and dense
initial spatfalls.

With the post setting rates of survival postulated, in
the absence of dredging the first brood contributes 3.8% of
the 500 spat surviving at the beginning of winter. The second
contributes 35% and the third 61 .2%.

The effect of dredging on the bar or sample is seen
by multiplying the complement of the reduction in survival
rate attributable to dredging by the percent contribution each
brood makes to the bar or sample. Thus dredging during the
respective spawnings would have the following effects:

(1-.514)(.0380) = 0.0185 for first spawning;

(1-.1 19)(.35) = 0.3084 for the second spawning;

(1-.307)(.612) = 0.4241 for third spawning.

Sum 0.7510

If dredging takes place during all three spawnings, the
reduction in spat reaching winter would be approximately
25% ( = 1 - .7510). If dredging takes place in only one of the
spawning periods, the reduction in spat reaching winter
would be the sum of the two non-dredging periods’ percent
contributions, plus the resultant from the above multiplica-
tion for the affected period. If dredging occurs only during
the first period the reduction would be (1 - (.0185 + .35 +
.612) = 1.95%. Similarly, for the second period reduction
would be (1- (.0380) + .3084 + .612) = 4.16%, and for the
third period, (1- (.0380 + .35 + .4241) = 18.79%.

RELATIONSHIP OF CURRENT STATUS
OF MARYLAND OYSTER RESOURCES
TO CONCLUSIONS OF THIS PAPER

There are few, if any, oyster bars in Maryland that cur-
rently approximate the reproductive stability assumed for the
hypothetical population discussed in this paper. Meritt (1977)
noted that over the past decade, reproductive success on all
oyster bars has been dramatically reduced. He noted that
there had been two extended periods of low natural reproduc-
tion in the past, from 1952 through 1960 and from 1966
through 1975, with the second period including the effects
of hurricane Agnes (June, 1972). Meritt stated, however, that
the problem of reproductive failure pre-dated the storm. He
concluded that a major, Bay-wide decline in recruitment had
occurred since 1965, and that the prognosis for a recovery
to levels of spat set that had characterized the early 1960’s
was poor, even under good environmental conditions. He sug-
gested that the paucity of brood stock currently existing (in

10

LARVAL ENTRAINMENT

1 975-1 977) could prevent the species’ being able to take ad-
vantage of favorable conditions.

Krantz (1985) reiterated that for over a decade there
has been a serious decline in reproduction and survival of
oysters in Maryland. He indicated that there is a poor
likelihood that oyster management can reverse the effects
of the decline, given that shell planting and seed moving
worked well when the Bay-wide average spatfall was greater
than 80 per bushel and the seed collecting areas received
a spatfall of 1 ,000 to 2,000 spat per bushel. He noted that
not only have the densities of spat setting fallen, but the
number of areas used for planting shell or collecting seed
has contracted. The situation posed by this combination of
factors has been aggravated by an outbreak of Haplosporidia
nelsoni (Haskin, Stauber and Mackin)(MSX) in 1980-1983,
which offset the anticipated improvements from the good
spatfall years of 1980-82. Krantz noted that the Maryland
oyster fishery is now dependent upon the annual levels of
spatfall. That is, harvest in each year now depends upon a
single year class, i.e., that which becomes three years old
in the given year.

That situation means that reproduction depends on a
brood stock that is one, two and three years old; there are
very few surviving individuals of ages four through seven that
could augment the production of larvae. Further, recruitment
in 1983 and 1984 was extremely poor. Even in the normally
reliable spat setting area of Broad Creek, a tributary of the
lower Choptank River, setting was g- 3atly reduced. Given that
the oyster fishery, much contracted though it is, now depends
upon the newly legal three year olds, this implies that the
harvest year 1986-1987 could be the last year in which a com-
merical oyster fishery could economically exist. Since the
modelling discussed in this paper postulated stable reproduc-
tion that depended upon spat sets in excess of what is
realistically the situation, and since even in the hypothetical
circumstances it appears possible that dredging could have
significant local effects, it appears reasonable to consider that
those effects are portrayed conservatively.

I conclude that a protracted dredging operation dur-
ing the spawning season near an oyster bar could have
deleterious effects on the viability of a bar. I therefore sug-
gest that non-dredging periods near oyster bars continue to
be a prudent technique of environmental management.

ACKNOWLEDGMENTS

I am indebted to Dr. George Krantz for the long discussions
we held on the subject of his paper, and for the information on size
and age of oysters that allowed me to make projections. His work
on the current situation in Maryland oysters provided the principal
means of showing that my assumptions of stability are conservative.

I also thank Dr. Victor Kennedy of Horn Point Environmental
Laboratory for pointing out pertinent literature and providing data
from literature and for discussing the concept of this paper. Without
his summarizations of existing literature, and his information on sex

ratios, I couid not have written this paper. Mr. Fred Kern, NMFS,
Oxford Laboratory, provided important data on larval densities, and
made literature available to me. Mr. Richard Reikenis and Mr. Marty
Snow gave me information on dredges and their performance that
made it possible to compute the conditional rate of fishing mortality.

LITERATURE CITED

Carriker, M.R. 1961. Interrelation of functional morphology, behavior,
and autoecology in early stages of the bivalve Mercenaria
mercenaria. Journal of the Elisha Mitchell Scientific Society
77:168-241.

Galtsoff, P. S. 1964. The American Oyster, Crassostrea virginica
Gmelin. Fishery Bulletin of the Fish and Wildlife Service
64:1-480.

Haskin, H. H. and S. Tweed. 1976. Oyster setting and early spat sur-
vival at critical salinity levels on natural seed oyster beds of
Delaware Bay. Final Report, O.W.R.T. Project B-037-NJ.
Water Resources Research Institute, Rutgers University.
Kennedy, V. S. 1980. Comparison of recent and past patterns of
oyster settlement and seasonal fouling in Broad Creek and
Tred Avon River, Maryland. Proceedings National Shellfisheries
Association 70:36-46.

Kennedy, V. S. 1983. Sex ratios in oysters, emphasizing Crassostrea
virginica from Chesapeake Bay, Maryland. The Veliger
25(4):329-338.

Kennedy, V. S. and Linda L. Breisch. 1981. Maryland's oysters:
research and management. University of Maryland Sea Grant
Publication No. UM-SG-TS-81-04.

Korringa, P. 1941. Experiments and observations on swarming
pelagic life and setting in the European flat oyster, Ostrea
edulis Archives A/eerlandaises de Zoologie 10:1-249.

Krantz, George E. 1985. Preliminary review: Current status of
Maryland’s oyster resources. Maryland Department of Natural
Resources, Tidewater Administration, Fisheries Division, An-
napolis, Maryland.

Lunz, John D. (unpubl., March 1985). An analysis of available infor-
mation concerning the entrainment of oyster larvae during
hydraulic cutterhead dredging operations with commentary
on the reasonableness of seasonally restrictive dredging win-
dows. U.S. Corps of Engineers; Coastal Ecology Group, En-
vironmental Resources Division, Environmental Laboratory,
Waterway Experiment Station, Vicksburg, Mississippi.
Meritt, D. W. 1 977. Oyster spat set on natural cultch in the Maryland
portion of the Chesapeake Bay (1939 - 1975). University of
Maryland Horn Point Environmental Laboratory UMCEES
Special Report no. 7. 30 pp.

Ricker, W. E. 1975. Computation and interpretation of biological
statistics of fish populations. Bulletin of Fisheries Research
Board of Canada: 191. Dept of the Environment, Fisheries and
Marine Service. Ottawa, Canada.

Shaw, W. N. 1969. Oyster setting in two adjacent tributaries of
Chesapeake Bay. Transaction of the American Fisheries Socie-
ty 98(2):309-314.

Webster, J. R. and W. N. Shaw. 1968. Setting and first season sur-
vival of the American oyster, Crassostrea virginica, near Ox-
ford, Maryland, 1961-62. United States Fish and Wildlife Ser-
vice Special Scientific Report no. 567.

EVALUATING THE NEED FOR DREDGING RESTRICTIONS
DUE TO OYSTER LARVAE ENTRAINMENT

H. GLENN EARHART
U. S. ARMY CORPS OF ENGINEERS
BALTIMORE DISTRICT
P. O. BOX 1715

BALTIMORE, MARYLAND 21203-1715, U.S.A.

ABSTRACT

The Baltimore District of the U. S. Army Corps of Engineers has responsibility to insure naviga-
tion for approximately 100 congressional^ authorized projects within the Chesapeake Bay region (Fig.
1) (U. S. Army Corps of Engineers, 1980). Hydraulic dredging costs were reviewed to evaluate the
economic impacts of the “time-of-the-year” restriction of 1 dune through 30 September for dredging
in shellfish areas to protect oyster larvae from entrainment during the dredging process. In Fiscal
Year 1985, the Baltimore District hydraulically dredged 8 projects which resulted in the removal of
499,404 cubic meters (m3) at a cost of $2,518,377 or $5, 05/m3. Only one project was dredged during
the summer with 1 15,000 m3 of dredged material removed at a cost of $3.42/m3. The remaining pro-
jects were completed in the fall, winter, and spring, and ranged in cost from $4.1 2/m3 to $1 5.94/m3.
In the absence of site specific data, an evaluation of the physical and biological characteristics of
the dredging process, shellfish larval behavior, sediment chemistry and current velocities leads to
the conclusion that entrainment associated with maintenance dredging of Federal projects is unlikely
to adversely impact oyster productivity in the Chesapeake Bay. When the scale of physical and biological
impacts are put in perspective with economic impacts on a project by project basis, a relaxation of
the summer dredging restriction due to entrainment would appear to be justified.

Due to an awareness of the current economic climate
and an increased emphasis on substantiating the Federal role
for public works projects, it is important to define the interest
the Baltimore District of the U. S. Army Corps of Engineers
has in this symposium. The following is an analysis of our
dredging program, examining economic biological and
physical impacts of the dredging process. The issue is the
summer dredging restriction of 1 June - 30 September im-
posed on our maintenance dredging program for the preven-
tion of entrainment of oyster larvae.

All dredging projects are congressionally authorized
with conditions in each authorization that outline specific pro-
ject dimensions. A majority of the project authorizations require
participation of a local sponsor prior to initiation of any work
by the Corps of Engineers. Usually, the local sponsor will re-
quest the Corps to dredge the Federal channel to create or
maintain commercial navigability and will agree to provide
suitable disposal areas, contribute dike construction costs
or any other specific requirement of the project authorization.
Consequently, it is imperative that the concerned resource
agencies verify the need for those environmental restrictions
that are costly to implement.

If scientifically valid environmental restrictions are
necessary to protect the resource, the Corps can then re-
spond to those who argue that the additional costs are un-
necessary to protect the resource. The Corps of Engineers
must justify additional costs based on the most current state
of knowledge. As a result, environmental restrictions, such
as a summer dredging restriction to protect oyster larvae en-
trainment, must be based on sound, technically defensible
information.

While the Baltimore District is responsible for approx-
imately 100 Federal navigation projects in the Chesapeake
Bay region, we recognize that the District boundaries include
abundant estuarine biological resources. To protect these
resources from potentially adverse impacts of the hydraulic
dredging process, we have implemented time-of-the-year
dredging periods or “windows” to protect the specific
biological resource located in a given project area.

Historically, we have not been permitted to dredge in
oyster producing areas from 1 June through 30 September
to protect spawning oysters and from 15 December through
28 February to protect adult oysters from sedimentation and
reduced filtering capacities during low water temperatures

American Malacological Bulletin, Special Edition No. 3(1 986): 1 1-16

11

12

LARVAL ENTRAINMENT

# Project Locations

Fig. 1. Baltimore District project locations.

RESTRICTED DREDGING
PERIODS*

'ACTUAL PERIOD WILL VARY DEPENDING ON WATER TEMPERATURES
AND OTHER ENVIRONMENTAL FACTORS.

Fig. 2. Environmental dredging windows for the important species
of the Chesapeake Bay.

(Fig. 2) (Pisapia, 1975). For projects located in the vicinity
of oyster bars (Fig. 3) (Lippson, 1973), we are permitted to
dredge from 1 October - 1 5 December (75 days) and 1 March
- 31 May (92 days).

Most of our projects are located in oyster bar areas,
a fact that illustrates a classic case of multiple uses of a water-
way. A significant number of our Federal channels that must
be maintained under dredging restrictions to protect oysters
are themselves justified by and used by watermen for access
to oyster bars and for transporting catch into port for
distribution.

With the exception of Baltimore Harbor maintenance
dredging, most of the dredging projects are accomplished
with a hydraulic cutterhead dredge. In effect, the dredge
removes bottom material and pumps it through a pipeline to
a disposal site. The operational activity that underlies the en-
trainment issue with respect to hydraulic dredging is the ac-
tion of the cutterhead and the suction of the sediment/water
slurry (Fig. 4) (Huston, 1970). The cutterhead is lowered into
and rotated to loosen the sediment. The sediment and water
are then pumped through the pipeline to the disposal area.
Optimally, the slurry is 80% water and 20% solids, a variable
determined by the efficiency of the dredge (Huston, 1970).

Output of the dredge (sediment and water pumped
through the pipe) is a function of discharge velocity, diameter
of pipe, length of pipeline, horsepower, efficiency of the
dredge, and quantity and depth of material. For example, a
typical discharge velocity for dredges working in Chesapeake
Bay is 4 m/sec with a 30 cm pipe so that the output is 1 ,086
m3/hr (Huston, 1970). Because the normal condition is to
pump 20% solids, 21 7 m3 of this volume would be solids and
the remainder water.

Table 1 presents budget data for the National Corps
of Engineers Fiscal Year (FY) 1985 dredging program that
resulted in the removal of approximately 191 .1 million m3 of
dredged material by non-hopper dredges at a cost of $444. 1
million, or $2. 32/m3 (U. S. Army Corps of Engineers, North
Atlantic Division, 1985). In FY 1985, the Baltimore District
dredging program, which includes clamshell as well as
hydraulic dredging, removed approximately 2,765,839 m3 of
dredged material for $18,914,327 or $6. 83/m3 (U. S. Army

Table 1. FY 85 Corps of Engineers national dredging program.

Cubic Meters
(x 1 ,000)

Dollars
(x 1,000)

Contract Hopper Dredge
Government Hopper Dredge

19,877

26,292

77,837

49,755

Total Hopper Dredge

46,169

127,592

Contract Non-Hopper Dredge
Government Non-Hopper Dredge

167,114

24,008

410,726

33,401

Total Non-Hopper Dredge

191,122

444,127

TOTAL ALL DREDGING

237,291

517,719

EARHART: DREDGING RESTRICTIONS

13

Fig. 3. Location of oyster producing areas in Chesapeake Bay.

Corps of Engineers, Baltimore District, 1985).

Because the entrainment issue is specifically related to
the hydraulic cutterhead dredging process, I have summar-
ized the Baltimore District’s FY 85 program for hydraulic dredg-
ing projects that entails dredging of approximately 499,000 m3
(Table 2). The remainder of the Baltimore District’s total pro-
gram yardage volume is accounted for by the Baltimore Har-
bor project, usually done with a bucket dredge. The competi-

tion for program dollars nationwide is intense. Cost efficien-
cy is therefore a very important program objective. For ex-
ample, a cost reduction of $1 ,20/m3 for more than 2.7 million
m3 of dredged material removed in our District alone would
yield a significant cost savings.

To further illustrate the impacts of summer dredging
restrictions, the FY 1985 program was summarized compar-
ing dredging season to cost/m3 (Table 3). Due to favorable

14

LARVAL ENTRAINMENT

Fig. 4. Cross-section view of typical cutterhead suction dredgehead.

working conditions, the summer project yielded the most
favorable costs. In the summer of 1985, Philadelphia District
dredged the New Jersey intercoastal waterway at a cost of
$2. 09/m3. Norfolk District opened bids for a project on the
James River scheduled for the summer at a cost of approx-
imately $2. 68/m3 (U. S. Army Corps of Engineers, WRSC,
1985). Thus the pattern of generally lower summer costs is
consistent.

There are many reasons that account for differences
in cost of dredging based on season. Dredging contractors
estimate that it takes 40% longer to do the same work in the
winter compared to the summer. Plant repairs, pipeline
relocations and worker safety and productivity are drastical-
ly reduced in winter. Another critical economic factor is ice.
Because it is difficult to predict freezing conditions in
Chesapeake Bay, contractors inflate bid prices to buffer a
potential early freezing and resultant lost time. Also, engines
must be kept constantly running to prevent freezing of
freshwater lines.

Another problem area that indirectly affects the
economics of the dredging program is contract administra-
tion. All of the District’s work is done by private contract dredg-
ing companies through the competitive bidding process. The

Table 2. Baltimore District’s FY 85 hydraulic cutterhead dredging
program.

Project

Cubic Meters

Cost

$/M3

Season

(x 1,000)

$(x 1,000)

Honga

119

490.0

4.12

Fall

Pocomoke

51

266.4

5.23

Winter

Wicomico

103

532.3

5.17

Fail

Little Wicomico

7

111.6

15.94

Spring

Knapps Narrows

30

179.0

5.97

Spring

Queenstown

28

224.0

8.00

Spring

Susquehanna

115

394.0

3.42

Summer

Ocean City

46

321.0

6.98

Spring

TOTAL

TOTAL

AVERAGE

499

2,518.3

5.05

Table 3. Baltimore District’s FY 85 hydraulic dredging program com-
paring cost per cubic meter to dredging season.

No. of Projects

Cubic Meters

Cost

$/M3

(m3)

<$)

FALL

2

221,712

1,022,300

4.61

WINTER

1

51,223

266,427

5.23

SPRING

4

1 1 1 ,788

835,650

7.47

SUMMER

1

114,678

394,000

3.42

cost of the job is contingent upon what is advertised in the
bid specifications. The specifications tell the contractor how
to do the work and what will be the resulting costs. The work
is usually advertised 3 months prior to actual dredging. Due
to the short time frame, the environmental dredging window
must be established in advance while providing adequate pro-
tection for the resources in the area.

For example, in order to dredge 115,000 m3 of
material, the contractor must average 1 ,500 m3/day to com-
plete the job within the 76-day period of 1 October to 15
December. Because a typical dredging contractor averages
1 ,500 m3/day, we can expect to receive higher $/m3 costs
to do that dredging because there is no allowance for lost
time or weather delays. Also, if the contractor does not finish
on time, he will have to come back in the next environmen-
tally acceptable dredging period. This will result in additional
mobilization and demobilization costs and lost time waiting
for the window to open. The end result is that the contrac-
tors will inflate their bids to account for the potential problems.

Evaluating quantative economic impacts is a much
easier exercise than defining the characteristics of larval
oyster behavior and potential entrainment during hydraulic
dredging. As is usually the case in this type of issue with a
notable lack of scientific documentation, regulatory, manage-
ment and academic professionals are required to make sub-
jective decisions concerning the management of the
resources. It is within this context that I present my
arguments, based on existing data on larval behavior and the
hydraulic dredging process, to propose that dredging does
not significantly impact oyster productivity by entrainment of
larvae in the Chesapeake Bay.

The average life cycle of oyster larvae is between 2-3
weeks with some variation depending on environmental con-
ditions (Galtsoff, 1964), The predominant time for spawning
in the Chesapeake Bay has been reported to be June through
August with a marked peak in setting occurring between late
June and September, typically during July (Beavins, 1954;
Kennedy, 1980). Oyster larvae can be characterized as “free-
swimming”, with velocity, direction and distance of travel
determined by numerous factors. Most early stage larvae in
James River were distributed randomly throughout the ver-
tical water column (Andrews, 1983). More importantly, when

EARHART: DREDGING RESTRICTIONS

15

the larvae reach the late umbo state (2-3 days prior to set-
ting) they were likely to be found near the bottom and “ac-
tively” select substrata suitable for setting (Andrews, 1983).
Logically, the only larvae that will be entrained are those that
are on the bottom and located in the vicinity of the cutterhead
during the dredging process.

Historically, maintenance dredging occurs in those
areas where erosion and sedimentation cause shoaling. The
composition of the channel sediments that result in frequent
shoaling are usually silts, clays and fine-grained sands (Bar-
tos, 1977). As a result, oyster setting in frequently dredged
channels is unlikely. In the absence of dredging, any late
stage larvae will detect the unsuitable bottom substratum and
search for suitable setting bottom. Any late stage larvae that
set in the channel will be lost to the system regardless of the
dredging process. Also, it has been reported that oysters do
not grow well in areas that are subject to shoaling or sedimen-
tation (Haven and Whitcomb, 1983).

Horizontal movement in the water column by larvae
is dependent on patterns of tidal and current circulation, water
column density stratification and surface mixing. It is intuitive-
ly obvious that water current circulation provides the prime
mode of movement. Because frequently dredged channels
are deeper than surrounding areas, current velocities vary
in the channels. Current velocities in the channel were
measured by the Baltimore District for the Chester River,
Maryland. They ranged from 0.3 to 0.5 m/sec at a depth of
1.0 m, 0.5 m/sec at 3m, and 0.2 to 0.3 m/sec at 6m (U. S.
Army Corps of Engineers, Baltimore District, 1980). Flood tide
currents have been measured at the entrance to Chesapeake
Bay at 0.5 m/sec compared to 0.3 m/sec in the vicinity of the
Chesapeake Bay bridge. Ebb tide currents have been
reported at 0.8 m/sec and 0.4 m/sec, respectively (Corps of
Engineers, Baltimore District, 1981). The upper sections and
bottlenecks of the Chesapeake Bay can have increased cur-
rent velocities. Local precipitation and runoff can also cause
current velocities to increase or decrease above or below nor-
mal ranges (Lippson, 1973).

Discharge velocities of approximately 4 m/sec for 30
cm hydraulic dredges were reported by Huston (1970). Mr.
F. Hall of Cottrell Engineering Corporation reports suction
velocities for their dredge “Richmond” as being approximate-
ly 60% less than the discharge velocity (personal communica-
tion). Suction velocities of 1-2 m/sec less than discharge
velocities were also reported by Huston (1970). As a result,
cutterhead suction velocities ranged from 2.0 - 3.0 m/sec com-
pared to natural current velocities ranging from 0.2 - 0.8
m/sec.

Average water contents for sediments removed dur-
ing maintenance dredging is 70% and higher (Halka and
Zoltan, 1984). As previously discussed, a typical hydraulic
dredge pumps a slurry composed of 80% water and 20%
solids. Because maintenance dredging involves removing
recently deposited material, that material in Maryland’s part
of the Chesapeake Bay is about 20% solids and 80% water
as it sits on the bottom ( Pritchard, 1985). Consequently, when
the cutterhead is lowered into the sediment, a significant
volume of water required for the slurry is derived from the

sediments. As a result, less water is entrained from the water
column to compose the dredge slurry reducing the probability
of entraining late umbo stage oyster larvae during the dredg-
ing process. In fact, the impact zone on the bottom where the
oyster larvae must be found to be entrained is small
regardless of the particular larval stage or the probability of
being located on or near the bottom.

The State of Maryland has been very flexible and
cooperative in granting extensions to the windows. Unfor-
tunately, the State finds itself in a precarious situation
because it is under significant political pressure from boating
interests to alter the State’s preferred recommendations to
get the job done. Normally, we try to accommodate the con-
tractor in trying to reduce his potential losses, while trying
to obtain an acceptable channel expeditiously for the com-
mercial interests. If a valid interest is being served and the
dredging process adversely entrains oyster larvae, then the
program should incur the added costs for environmental pro-
tection. On the other hand, a broad window such as 1 June
to 30 September to protect from entraining larval oysters that
have a 3-4 week larval period of which the larvae are located
near the bottom for only 2-3 days of the entire period seems
excessive.

CONCLUSION

The Corps of Engineers recognizes a need and a
responsibility to define the impacts of entrainment caused
by hydraulic dredging. Resource managers must make deci-
sions frequently without specific documentation of entrain-
ment impacts. A “nice to have” summer dredging restric-
tion has significant economic impacts relative to a “need to
have” restriction as perceived by many. It is unlikely that
hydraulic dredging adversely impacts oyster larvae in con-
sidering water content of the maintenance dredging
sediments, the limited vertical zone of impact, the relatively
short term impacts of the dredging process, aspects of lar-
val behavior and the physical dynamics of the water column.
The Corps of Engineers is not advocating abolishment of
necessary environmental protection restrictions such as
dredging windows imposed on our dredging program.
However, evaluating the existing data as it specifically per-
tains to the hydraulic maintenance dredging process indicates
that the 1 June to 30 September summer dredging restric-
tion to protect from entraining oyster larvae is excessive.

LITERATURE CITED

Andrews, Jay D. 1983. Transport of bivalve larvae in the James River,
Virginia. Journal of Shellfish Research 3:29-40.

Bartos, Michael J. 1977. Classification and engineering properties
of dredged material. Technical Report D-77-18, U. S. Army
Engineer Waterways Experiment Station, Vicksburg,
Mississippi. 119 pp.

Beavin, Francis. 1954. Various aspects of oyster setting in Maryland.

Proceedings of the National Shellfisheries Association 45:29-37.
Galtsoff, Paul S. 1964. The American Oyster. Fishery Bulletin of the
Fish and Wildlife Service 64:355-380.

16

LARVAL ENTRAINMENT

Halka, Jeffrey and Nicholas Zoltan. 1984. Final Report - Monitoring
of dredged material disposal from dredging the approach
channels to Baltimore Harbor between December 1982 and
April 1983. Maryland Geological Survey, Baltimore, Maryland.
70 pp.

Haven, Dexter S. and James P. Whitcomb. 1983. The origin and ex-
tent of oyster reefs in the James River, Virginia. Journal of
Shellfish Research 3:141-151.

Huston, John. 1970. Hydraulic Dredging. Cornell Maritime Press, Inc.,
Cambridge, Maryland. 318 pp.

Kennedy, Victor S. 1980. Comparison of recent and past patterns
of oyster settlement and seasonal fouling in Broad Creek and
Tred Avon River, Maryland. Proceedings of the National
Shellfisheries Association 70:36-46.

Lippson, A. J. 1973. The Chesapeake Bay in Maryland: An atlas of
natural resources. Johns Hopkins University Press, Baltimore,
Maryland. 55 pp.

Pisapia, Ralph C. 1975. Memorandum regarding spawning times for
Chesapeake biota (limited). Department of the Interior. U. S.
Fish and Wildlife Service, Annapolis, Maryland. 6 pp.

Pritchard, Donald W. 1985. Entrainment of larval oysters during

hydraulic cutterhead dredging operations. Working draft pro-
ceedings, Workshop on Entrainment of Larval Oysters. Col-
lege of Marine Studies, University of Delaware, Lewes,
Delaware. 160 pp.

U. S. Army Corps of Engineers, Baltimore District. 1980. River and
Harbor Project Map Book, Baltimore, Maryland.

U. S. Army Corps of Engineers, Baltimore District. 1980. Current
Velocities and Potential for Sediment Movement in the Chester
River, Queen Annes County, Maryland. Navigation Branch,
Baltimore, Maryland. 12 pp.

U. S. Army Corps of Engineers, Baltimore District. 1981 . Main Report
and Environmental Statement, Baltimore Harbor and Chan-
nels, Maryland and Virginia. Baltimore, Maryland.

U. S. Army Corps of Engineers, Baltimore District. 1985. FY 85
Budget Data, Navigation Branch, Baltimore, Maryland.

U. S. Army Corps of Engineers, North Atlantic Division. 1985. FY
85 dredging statistics. North Atlantic Division, NADCO-OP,
New York, New York.

U. S. Army Corps of Engineers, Water Resources Support Center.
1985. Post-Bid Data Base, Fort Beivoir, Virginia.

THE PUBLIC OYSTER BOTTOMS IN VIRGINIA:

AN OVERVIEW OF THEIR SIZE, LOCATION, AND PRODUCTIVITY

DEXTER S. HAVEN
and

JAMES P. WHITCOMB
VIRGINIA INSTITUTE OF MARINE SCIENCE
AND SCHOOL OF MARINE SCIENCE
THE COLLEGE OF WILLIAM AND MARY
GLOUCESTER POINT, VIRGINIA 23002, U. S. A.

ABSTRACT

The location size and extent of Virginia’s public oyster grounds was determined using a long
pole to probe the bottom, a towed sonic device that detected shell or oysters, and by sampling the
bottom with patent tongs for shell and oyster density. Station location was determined using an elec-
tronic positioning system (Raydist® ). Bottoms were classed as oyster reefs, mud-shell or sand shell
(productive or potentially productive). Areas having mud or sand or those in deep water over 30 ft.
(9.1 m) were considered unproductive. Average oyster harvest for seed and market size oysters over
the last ten years for various areas is related to size and location of various bottom type.

About 203,404 acres out of the total of about 243,000 acres of public bottom were surveyed;
about 21 .8% of the surveyed areas was classed as productive or potentially productive. Average pro-
duction was low on most of these public bottoms and ranged from 84.4 bushels/acre in the Great
Wicomico River to only 1.6 bushels/acre in the York River.

The seven areas producing the most seed and market oysters in terms of their average annual
production in Virginia bushels were: James River 432,1 71; Rappahannock River 146,999; Pocomoke
and Tangier Sounds 86,150; Sea side of Eastern Shore 63,122; Great Wicomico River 41,622;
Piankatank River and Milford Haven 39,024; and Mobjack Bay 29,730.

This paper gives an overview of the location, size and
productivity of the public oyster grounds (Baylor Grounds)
in Virginia, in preparing this paper, data were summarized
from an extensive study carried out from 1976 to 1981 titled:
“The Present and Potential Productivity of the Baylor
Grounds in Virginia (Vols. I and II) (Haven et al., 1981a). A
portion of this report relating to the James River, Virginia,
has been published (Haven and Whitcomb, 1983), and
reference can be made to the original report and the latter
publication for additional details on the composition of the
various types of oyster bottoms, salinity of the various grow-
ing areas, setting data, information on predators and
diseases, and the best use of various areas for molluscan
culture.

In discussing the overall productivity of Virginia for
oysters, it is pertinent to review historical landing data for the
state. Prior to 1960, Virginia was the leading oyster producer
in the United States, but since 1961 it experienced a major
decline in total production. To understand the reason for the
decline we must first recognize the dual nature of the state’s

industry which is based on the public sector managed by the
State, and the private sector managed by individuals or com-
panies. If has been the major decline in production from the
leased bottoms that has been responsible for the major
decline in total statewide production. Production from the
public bottoms has also declined, but until recently its pro-
duction relative to that from the leases has always been much
lower.

No detailed studies have been made on the produc-
tivity of individual leases or their suitability for oyster culture.
Therefore, this paper deals with public bottoms where data
are available. It includes information on the location of the
most important growing areas, their recent productivity, and
acreages of productive and potentially productive bottoms
in various estuaries.

PUBLIC AND PRIVATE SECTORS

The naturally productive public oyster bottoms in
Virginia were charted through a survey by Lt. J. B. Baylor
(hence referred to as the Baylor Survey) and were set aside by

American Malacological Bulletin, Special Edition No. 3(1 986): 17-23

17

18

LARVAL ENTRAINMENT

Fig. 1. Location of public oyster grounds (Baylor Grounds) in Virginia. Shaded areas are public clam grounds.

HAVEN AND WHITCOMB: PUBLIC OYSTER BOTTOMS

19

OYSTER SEASON

Fig. 2. Landing of market sized oysters in Virginia from 1947 to 1984. Data for Virginia bushels.

legislative action for public use (Baylor, 1894). Changes and
additions to the public ground boundaries have been made
since, and today they total about 243,000 acres. Data relative
to the industry, the decline in production, and the onset of
the oyster disease MSX Haplosporidium (Minchinia) nelsoni
(Haskin, Stauber and Mackin) (Andrews, 1 968) are summar-
ized below from an earlier publication (Haven et al., 1981a).

The state’s Baylor Grounds contain most of the areas
where oysters set and grow naturally, but they also contain
large areas of unproductive bottom (Fig. 1). These public
oyster bottoms are managed by the Virginia Marine
Resources Commission (VMRC), which collects statistical
data, enforces fisheries laws and regulations, regulates
season of harvest, collects various taxes on oysters and
license fees from harvesters and dealers, and plants shells
or seed oysters on public bottoms to enhance oyster produc-
tion. Salinities on most of these bottoms range from about
5 to 20 °/00.

Bottoms located outside designated public bottoms
can be leased from the VMRC by individuals or companies
for renewable 10 year periods. The annual leasing fee in
Chesapeake Bay is $0.75 per acre per year and $1 .50 in all
other locations. Salinities on leased bottoms usually range
from about 5 to 30 °/00- During the 1950s total lease size
ranged from about 100,000 to about 125,000 acres; in 1984
about 110,000 acres were leased. Most leased bottoms are
not naturally productive, as are many public bottoms, and
most must be planted with seed oysters to make them pro-

ductive. For the last 100 years over 75% of this seed has been
tonged from the public bottoms in the James River. Therefore,
the private sector is largely dependent on this source of seed
for its basic production (Haven et al., 1981b).

THE MAJOR DECLINE IN PRODUCTION

In Virginia the decline in oyster production began in
1960 when the oyster pathogen Haplosporidium (Minchinia)
nelsoni commonly known as MSX entered Chesapeake Bay
and caused high mortalities in the high-salinity growing areas
(Andrews, 1968). In the decade before 1960, annual produc-
tion of market oysters in Virginia averaged about 3.2 million
Virginia bushels.1 Of this total, about 0.55 million came from
Virginia’s 243,000 acres of public bottom, and 2.65 million
from about 125,000 acres of leased bottoms. That is, leased
bottoms were producing nearly 5 times as many oysters as
did the public bottoms and on fewer acres (Fig. 2). The reason
for this large difference in production was that leased areas
were usually planted with seed oysters from the James River
at rates ranging from 500 to 1000 bushels per acre. In con-
trast, harvest from the public bottoms originated from a
natural set, or from limited repletion efforts by the State
(Haven et a!., 1981b).

About 5 years after MSX became active total produc-
tion of market oysters fell sharply to about 1 .7 million bushels

’The Virginia oyster bushel is 1 .397 times larger than the U. S. stan-
dard bushel.

20

LARVAL ENTRAINMENT

Table 1. Acres of productive or potentially productive bottom, unproductive areas and average oyster production (1974-75
to 1983-84) in Virginia estuaries in Chesapeake Bay that may produce seed or market oysters.

River

system

Acres pro-
ductive or
potentially
productive

Unproduc-
tive acres

Total

acres

% produc-
tive bottom

Average
produc-
tion in
bushels

Total

market

plus

seed

James1

16,246

8,906

25,152

64.6

market

55,301

seed

376,870

Total

432,171

Piankatank &

Milford

839

7,327

8,166

10.2

Haven

market

17,223

seed

21,801

Total

39,024

Great

Wicomico

493

3,578

4,071

12.1

market

38,954

seed

2,668

Total

41,622

Corrotoman

213

1,348

1,561

13.6

market

6,401

seed

0

6,401

Total

17,791

21,159

38,950

1 1ncludes Production from Nansemond River.

with most of the decline due to the absence of production
from leased bottoms. The decline continued and by 1984 total
production was only 523,614 bushels; leased bottoms yielded
285,015 bushels, and public bottoms produced 238,614
bushels (VMRC, An. Rpts.).

MSX was the cause of the initial major decline in pro-
duction from leased bottoms after 1960 since many large
leases were in high salinity areas. Since that time, the con-
tinued decline has been associated with the persistence of
MSX, adverse economic conditions related to costs of grow-
ing and harvesting oysters, and failure of the private sector
to utilize modern technology. The public oyster grounds ex-
isting in low salinity areas have also been influenced by MSX
but to a lesser degree (Haven et a!., 1981b).

MATERIALS AND METHODS

BOTTOM TYPES

The nature of the bottom was determined at nearly
250,000 stations by an experienced operator who used a long
pole (probe) to determine bottom composition by the poles
impact as it touched shell, sand or mud. An underwater
microphone was towed over the bottom between stations and
its amplified sound, as it impacted shell or oysters, was
recorded to supplement information obtained with the probe
(Haven et a /., 1979). Additionally, many thousands of samples
of the bottom were obtained with patent tongs to record oyster
and shell densities (Haven et at., 1981a). Each probe of the

bottom was made at a known position which was established
by electronic positioning gear, Raydist® , which showed the
vessels location in terms of a parabolic grid system referenced
to latitude and longitude. Using these data, the Baylor bot-
toms were classified as sand, gravel, mud, shell-mud, shell-
sand or shell-oysters (hard oyster reef). Subsequently, the
boundaries of the various bottom types were outlined on
charts. Areas were determined with a digitizing planimeter
(Haven et al., 1979 and Haven et a!., 1981a).

Firm bottoms (oyster reefs) largely composed of a
shell-oysters mixture are rated as having the highest value
for oyster culture. The less firm bottoms composed of shell-
mud or shell-sand are next in value. They may or may not
have living oysters and are classed as productive or poten-
tially productive. Bottoms lacking shell or those composed
of sand, mud or gravel and those in waters exceeding about
10 rn are considered to have a low potential for growing
oysters (unproductive).

The sonic gear and bottom probe did not differentiate
between living oysters and non-living oyster shell; however,
we do not regard this distinction of major importance in our
formulation of a classification, i.e., productive, potentially pro-
ductive, or unproductive. The presence of shell material (live
oysters or shells) showed the present state or past history
of a bottom. If “shell” (live oysters or non-living shell) is
detected, it meant that oysters were growing there or did grow
there in the recent past.

Statistical data on landings of oysters in Virginia were
obtained from the Virginia Marine Resources Commission,

HAVEN AND WHITCOMB: PUBLIC OYSTER BOTTOMS

21

Table 2. Acres of productive or potentially productive bottom, unproductive areas and average market oyster
production (1974-75 to 1983-84) in Virginia estuaries where market oysters are grown.

River

system

Acres produc-
tive or poten-
tially productive

Unproduc-
tive acres

Total

acres

% produc-
tive bottom

Average
production
in bushels

Poquoson1

808

8,123

8,931

9.0

5,949

York

1,089

1,359

2,449

44.5

1,729

Mobjack Bay
Mobjack Bay

412

5,197

5,609

7.3

29,730

Tributaries2

195

2,258

2,453

7.9

9,283

Rappahannock
Back & Little

9,502

33,191

42,693

22.2

146,999

Wicomico
Potomac River

“

—

—

—

5,627

Tributaries3

Total

817

12,823

1,951

52,079

2,768

64,902

29.5

6,914

includes large areas in Chesapeake Bay.

includes East, North and Ware and Severn Rivers (about 5% of Baylor Areas not included),
includes Nomini, Lower Machodoc, Goan and Yeocomico Rivers; Currioman not surveyed.

Newport News, Virginia (VMRG, 1947-1984). Statistical data
on landings (production) are based on the biological year
which extends from 1 October to 30 June. These data were
averaged for a ten-year period (1974-75 to 1983-84).

Areas in this paper are given in acres since this is the
official method used by Virginia to measure size of oyster
grounds. The conversion factor is: 1 acre = 0.4047 hectacres.

RESULTS

SHAPE OF PRODUCTIVE OR POTENTIALLY
PRODUCTIVE BOTTOMS

Most of the areas of shell-oyster bottoms within the
Baylor Survey bottoms in Virginia occur at depths between
the 6 and 18 feet (1 .8 - 5.5 m) contours as shown on NOAA
charts. In a few instances shells or oysters occur as deep
as 30 feet (9.1 m). However, in the shallow lagoonal areas
on the seaside of the Eastern Shore, oysters occur intertidally,
or just below the low tide level.

Areas termed oyster reefs are not contiguous, but oc-
cur as irregularly shaped areas usually surrounded by shell-
sand or shell-mud bottoms. The reef areas occur in a definite
pattern in respect to channels and other features of the bot-
tom, and although they appear irregular in outline they may
be classed into four types: parallel, longitudinal, pancake and
transverse (Price, 1954; Scott, 1968; Haven and Whitcomb,
1983). The reef areas may vary in area from less than one
acre to several hundred.

OYSTER PRODUCTION, LOCATION AND SIZE
OF PRODUCTIVE BOTTOMS

Four of Virginia’s estuaries produce seed or market
sized oysters2 (Fig. 1; Table 1). Of these, the James has by
far the largest acreage of productive or potentially produc-
tive bottom (16,246 acres) and the highest average annual

production (432,171 bushels). Most of the production (376,870
bu. annually) consists of seed size oysters. The three remain-
ing estuaries have produced relatively little seed during the
1974-75 to 1983-84 period, but annual average market oyster
production has been substantial: 17,223 bushels in the
Piankatank River and 38,954 bushels from the Great
Wicomico River, and 6,401 from the Corrotoman River.

Most of Virginia’s estuaries on the western side of
Chesapeake Bay produce market oysters (Fig. 1; Table 2).
Usually these oysters originate from a natural set on naturally
occurring shell, or in some instances, from a natural set on
shell planted by the VMRC. Of the seven areas shown, the
Rappahannock River has by far the largest acreage of pro-
ductive or potenially productive bottom (9,502 acres) and the
greatest average annua! production (146,999 bushels). Mob-
jack Bay, with only 412 acres, ranked next in average an-
nual production with 29,730 bushels annually. The remain-
ing areas, with acreages ranging from 195 to 1,089 acres,
had relatively low average annual production. The percen-
tage of productive or potentially productive bottom within
these seven locations follows: Rappahannock River - 22.2%;
York River - 44.5%; Potomac River tributaries - 29.5%;
Poquoson River - 9.0%; and Mobjack Bay and tributaries -
7.3% and 7.9%, respectively.

Large acreages of Baylor bottom exist on the western
side of Chesapeake Bay off the entrances to the Rappahan-
nock, Piankatank, and Great Wicomico rivers (Fig. 1; Table
3). In this large area only 639 of the 27,247 acres (2.3%) are
classified as productive or potentially productive. Average an-
nual production of 22,884 bushels, is fairly high for the 639
acres of productive and potentially productive bottoms.

Estuaries on the eastern side of Chesapeake Bay in-
clude Pocomoke and Tangier Sounds and several small

2Three inches in length or longer.

22

LARVAL ENTRAINMENT

Table 3. Acres of productive or potentially productive bottom, unproductive acres and average market oyster
production (1974-75 to 1983-84) in two areas on the western side of Chesapeake Bay where market oysters
are grown.

River

Acres pro-
ductive or
potentially
productive

Unproduc-
tive acres

Total

acres

% productive
bottom

Average
production
in bushels

Off Entrance
to Piankatank

River

610

16,229

16,839

3.6

Off Entrance
to Great Wicomico
River

29

10,378

10,407

0.3

Total

639

26,607

27,246

22,884

Table 4. Acres of productive or potentially productive bottom, unproductive areas and average oyster pro-
duction (1974-75 to 1983-84) in Pocomoke - Tangier Sounds, several bay side Eastern Shore creeks.

River

Acres pro-
ductive or
potentially
productive

Unproduc-
tive acres

Total

acres

% produc-
tive bottom

Average
production in
bushels

Seaside of

Eastern Shore
market
seed

Total

7,226

33,587

40,813

17.7

32,314

30,808

63,122

Pocomoke &

Tangier Sound
market

5,875

25,096

30,971

19.0

86,150

The Bay Side

Tributary Creeks1
market

83

439

522

15.9

very low-land-
ings included
above

includes Pungoteague, Occohannock, and Nassawadox Creeks.

tributary creeks on the western side of the Eastern Shore
peninsula (Fig. 1 ; Table 4). The Seaside of the Eastern Shore
has a total of 40,813 acres, but only 7,226 (17.7%) is classed
as productive or potentially productive. Annual production for
the last 10 years from this location has been 63,122 bushels.
Pocomoke and Tangier Sounds also have large areas of
Baylor bottom (30,971 acres), but only 19.0% or about 5,875
acres has shell. From this large area annual production was
86,150 bushels. Productions from the three Bayside
tributaries has been very low (Table 4).

The preceeding data outlined the extent of the produc-
tive and potentially productive bottoms, the size of unproduc-
tive areas and average annual production. Table 5 sum-
marizes production in terms of bushels per acre. These data
show the low level of productivity of most of the estuaries.
The highest level of productivity (84.4 bu/acre) is for the Great
Wicomico River, and the next highest is 72.2 bu/acre for Mob-

jack Bay; both areas have had shells planted on them by the
VMRC in recent years, and it is probable that at least half
of the mature oysters originated from these plantings.

DISCUSSION

The total acreage surveyed in this study (Tables 1-4)
is 203,405 acres which is slightly less than the 243,000 acres
usually cited as the total acreage of Public Bottoms in Virginia.
That is, our study included about 84% of the State’s reported
acreage of Public Bottoms. Areas omitted from the study in-
clude sites now used for spoil disposal, areas now located
on dry land due to shoreline changes since 1894, some
shallow marsh areas on the Seaside of the Eastern Shore,
and certain small areas where the electronic positioning gear
would not operate. One large bottom area in the open part
of Chesapeake Bay to the north of the entrance to the Rap-

HAVEN AND WHITCOMB: PUBLIC OYSTER BOTTOMS

23

Table 5. Total oyster production related to total acres of productive
or potentially productive bottoms. Expressed as bushels per acres

(data from Tables 1 , 2,

3 and 4).

1. SEED AND MARKET AREAS (TABLE 1).

River

Acres

Average

Bushels

productive

production

oysters

or potentially

in bushels

per acre

productive

James

16,246

432,171

26.6

Piankatank &

Milford Haven

839

39,024

46.5

Great Wicomico

493

41,622

84.4

Corrotoman

213

6,401

30.1

II. MARKET GROWING AREAS IN ESTUARIES ON THE

WESTERN SIDE OF CHESAPEAKE BAY (TABLES 2 AND 3).

Poquoson

808

5,949

7.4

York

1,089

1,729

1.6

Mobjack Bay

Mobjack Bay

412

29,730

72.2

Tributaries

195

9,283

47.6

Rappahannock

Back & Little

9,502

146,999

15.5

Wicomico Rivers

—

5,625

—

Potomac River
Tributaries

Off Entrance to

817

6,914

8.5

Piankatank River

Off Entrance to

610

22,884

37.5

Great Wicomico River

29

—

—

IV. POCOMOKE-TANGIER SOUND (TABLE 4).

Seaside of

Eastern Shore
Pocomoke and

7,226

63,112

8.7

Tangier Sound

Eastern Shore

5,875

86,150

14.7

Tributary Creeks
(3) on Bay-side

83

Very low

pahannock River was not surveyed because of time limita-
tions; however, it was largely an unproductive sand bottom
(Haven et at., 1981a).

The study showed that statewide, 44,437 acres had
moderate to high potential for oyster culture, or 21 .8% of the
acreage surveyed. This does not mean that the remaining
78.2% have no value. Often, bottoms without shell may be
highly productive of hard clams, soft clams, fish or crabs.

The estuary which produced the most oysters was the
James River which averaged about 376,870 bushels of seed
and 55,301 bushels of market-sized oysters annually from
about 16,246 acres. The Rappahannock River ranks next to
the James River with an average annual production of
146,999 bushels from about 9,502 acres. Next in order of rank
are Pocomoke and Tangier Sound followed by the Seaside
of the Eastern Shore.

Care must be used in interpreting production data
presented in this study. In the James River, for example, the
present demand for seed is very low due to adverse economic

factors influencing the purchase of seed by lease holders.
It is believed that if demand increased, production could be
higher. In contrast to the James River, most of the remain-
ing areas surveyed produced relatively few oysters from their
productive and potentially productive bottoms. Yields from
these latter areas are probably as high as the areas can now
support under the present system of management. We
believe that with proper management and more funds for
repletion, many areas might yield more oysters. Suggested
improvements include the adoption of modern technology in
dredging seed and in planting shell, and in a more efficient
use of seed oysters produced in the James River.

ACKNOWLEDGEMENTS

The authors gratefully thank Messrs. Kenneth Walker, Paul
Kendall, Reinaldo Morales and others who assisted in the field
surveys of the public oyster bottoms. Also, many thanks are given
to Mr. Paul Kendall for tabulating much of the data on which the study
is based.

This study was financed in part by a grant from the National
Marine Fisheries Service through the Virginia Marine Resources
Commission (Contract No. 3-265-R-3).

This is Contribution No. 1306 from the Virginia Institute of
Marine Science, School of Marine Science, The College of William
and Mary, Gloucester Point, Virginia, 23062.

LITERATURE CITED

Andrews, J. D. 1968. Oyster mortality studies in Virginia. VII. Review
of epizootiology and origin of Minchinia nelsoni. Proceedings
of the National Shellfisheries Association 58(1967):23-36.
Baylor, J. B. 1894. Method of defining and locating natural oyster
beds, rocks and shoals. In: Oyster Records (pamphlets, one
for each Tidewater, Virginia, County, which listed precisely
the boundaries of the Baylor Survey). Board of Fisheries in
Virginia: 1 -770.

Haven, D. S., J. P. Whitcomb, J. M. Zeigler and W. C. Hale. 1979.
The use of sonic gear to chart locations of natural oyster bars
in lower Chesapeake Bay. Proceedings of the National Shell-
fisheries Association 69:11-14.

Haven, D. S., J. P. Whitcomb and P. Kendall. 1981a. The present
and potential productivity of the Baylor Grounds in Virginia,
Vols. I (167 pp.) and II (154 pp., 52 charts). Special Report
in Applied Marine Science and Ocean Engineering No. 243.
Virginia Institute of Marine Science, College of William and
Mary.

Haven, D. S., W. J. Hargis, Jr. and P. Kendall. 1981b. (revised). The
Oyster Industry of Virginia: Its status, problems and promise.
Virginia Institute of Marine Science, College of William and
Mary. Special Papers in Marine Science No. 4:1-1024.
Haven, D. S. and J. P. Whitcomb. 1983. The origin and extent of
oyster reefs in the James River, Virginia. Journal of Shellfish
Research 3(2): 141 -151.

Price, W. A. 1954. Oyster reefs of the Gulf of Mexico. Galtsoff, P.
S. Coordinator, Gulf of Mexico, its origin, waters, and marine
life. U. S. Fish and Wildlife Service Bulletin 89(55): 1 -491 .
Scott, A. J. 1968. Environmental factors controllling oyster shell
deposits. Texas Coast. From Fourth Forum on Geology of In-
dustrial Minerals, Austin, Texas: Univ. of Texas:131-150.
Virginia Marine Resources Commission Annual Reports 1947-1984.
Annual Reports to the Governor of Virginia 1947-1984, Rich-
mond, Virginia.

■

EXPECTED SEASONAL PRESENCE OF CRASSOSTREA VIRGINICA (GMELIN)
LARVAL POPULATIONS, EMPHASIZING CHESAPEAKE BAY

VICTOR S. KENNEDY
UNIVERSITY OF MARYLAND
HORN POINT ENVIRONMENTAL LABORATORIES

P.O. BOX 775

CAMBRIDGE, MARYLAND 21613, U.S.A.

ABSTRACT

The eastern oyster, Crassostrea virginica , exhibits spawning and settlement periods which vary
in onset and duration depending on latitude and local conditions. Shortest spawning periods (about
6-8 weeks) are found in summer (June - August) in the oyster’s northern habitat, with the period ex-
panding to include spring and autumn (March or May - October) further to the south of its range. In
general, regional spawning and spat settlement appear to be continuous, with one or more peak periods
(variable in time and space) occurring during the season. Thus, larval populations can be expected
to be present throughout the reproductive season. It is not clear what, if any, period of larval availability
is the most important determinant of successful recruitment. Thus, it is difficult to predict when activities
that might result in entrainment and mortality of oyster larvae could be permitted to occur during the
reproductive season.

Entrainment of pelagic or epibenthic larvae of
vertebrates and invertebrates in the water inflow into industrial
facilities has been of concern because of the often deleterious
results (e.g. Schubel and Marcy, 1978). The associated
mechanical, osmotic, or thermal stresses may be lethal, or
incapacitating to the point of making the affected organisms
more susceptible to predation or disease. In effect, the en-
training facility functions as a predator. This is also true of
dredge types used in estuarine sediment control (see Mohr,
1982 for review) in which the dredged material is deposited
either onshore or in specially constructed containment
systems. Entrained larvae can be killed by smothering, anox-
ia, starvation, or desiccation even if they survive the
mechanical forces caused by any pumping of the water-
sediment slurry that might occur.

Larvae of the eastern oyster, Crassostrea virginica
(Gmelin), are pelagic and are thus at risk of being entrained
during this phase of their life history, i.e. from fertilization to
settlement as spat. To estimate when larval populations might
be present, I review data on the extent and periodicity of C.
virginica spawning and settlement, especially for Chesapeake
Bay, the area of concern in this workshop. I also add new
quantitative data on weekly spawning for one region of
Maryland’s Chesapeake Bay in an attempt to determine if
there is some periodicity to gamete release that might be ex-
ploited in allowing dredging activity.

SPAWNING PERIODICITY

SEASONAL

Table 1 lists major reports of timing of oyster spawn-
ing from the most northerly location in which oysters can be
found to spawn on the North American east coast (Prince Ed-
ward Island) to the southerly end of their range in the Gulf
of Mexico. The period lengthens from about 6-8 weeks in the
north to about six months or more in the south. In the warm
waters of Hawaii to which C. virginica has been transplanted,
the spawning season is about eight months long.

In Chesapeake Bay the spawning period is in-
termediate between these cold and warm water extremes,
with the major spawning activity generally occurring from
June through September (Table 1; Kennedy and Krantz,
1982). Earlier in this century, spawning may have occurred
for a slightly longer period of time (Kennedy and Krantz,
1982). For example, Truitt (1929) indicated that data collected
from 1919-1929 revealed a pattern of spawning from early
June to early October.

If spawning period has indeed shortened in Maryland’s
Chesapeake Bay, the reasons for the change are not clear.
However, annual variations in environmental conditions can
have a considerable impact on the length of a spawning
season. As a case in point, in Spring 1 931 Bay waters warmed
up much earlier than usual (in late April - early May) with the

American Malacologicat Bulletin, Special Edition No. 3(1986):25-29

25

26

LARVAL ENTRAINMENT

Table 1. Spawning periodicity of Crassostrea virginica throughout
the species’ natural range and in Hawaii.

Location

Study

Period

Timing of

Spawning

Authority

Prince Edward
Island,

1961-62

June-August

Kennedy and
Battle, 1964

Canada

Long Island
Sound
Connecticut

1937-56

Start: June 26
to July 3

End (90%):Aug. 15
to Sept. 6

Loosanoff, 1965

Chesapeake

Bay,

Maryland

Not Stated

Early June-early
Oct.; peaks
usually in July

Chesapeake

Biological

Laboratory,

1953

1977-78

Late May-Sept.

Kennedy and
Krantz, 1982

Virginia

Many years

Mid June-early

Oct.

J.D. Andrews
(pers. comm.)

Gulf of Mexico,

Florida

1949-50

Late March-late
Oct., varying with
location; multiple
spawnings

Ingle, 1951

Not stated

May-October*

Butler, 1965

Hawaii

1963-65

Feb. -Nov.; mainly
March-October

Sakuda, 1966

* Crassostrea virginica transplanted earlier from Chesapeake Bay
spawned in Florida from May to October as did native Florida oyster
populations. In this the transplants were unlike Chesapeake Bay
populations which spawned from June - September during the same
experimental period (Butler, 1965).

result that spawning and settlement occurred in the Patuxent
River, MD in the middle of May, the earliest record in 13 years
of study (Truitt, 1931). Note also (Table 1) that Chesapeake
Bay oysters which had been transplanted to Florida and held
in local waters began spawning at the same time as did the
native Florida stock of C. virginica and spawned for a longer
period (May-October) than did oysters that same summer
(June-September) in Chesapeake Bay (Butler, 1965).

WEEKLY

Histological material collected approximately weekly
by Kennedy and Krantz (1982) on Deep Neck Bar in Broad
Creek, MD was re-examined using stereology (Lowe et al.,
1982) to determine if some cyclical pattern of gametogenesis
and spawning could be detected. Deep Neck Bar is an oyster
bar that has consistently experienced good spat recruitment.
The oyster tissue had been treated histologically by standard

procedures (Kennedy and Krantz, 1982) and included regular
mid-monthly samples taken on June 22, July 18 and August
14, for which one microscope slide had been prepared for
each oyster. For the additional weekly samples, the oysters
had been grouped three to twelve to a slide by processing
tissue from the dorsal body mass of each oyster rather than
a complete tissue cross-section of the body as was done mid-
monthly. Sample sizes ranged from 10 to 25 per week and
oyster size averaged 8.7 cm in length (hinge to bill).

To quantify the process of gametogenesis, the
histological slides were examined using a stereological techni-
que (Lowe et al., 1982; Kennedy and Van Heukelem, 1985).
The gonadal tissue was viewed through a microscope equip-
ped with an eyepiece reticule marked with a random array
of 42 points. The coincidence of a point with one of four tissue

7 17 12 8 15 16 4 15 15 13 7

I OO-r-T r

Fig. 1. Average volume fractions of four tissue components in the
gonadal region of Crassostrea virginica from Deep Neck Bar, Broad
Creek, MD in 1978. M.G. = Morphologically ripe gametes; D.G. =
Developing gametes; S = Spawning/spent tissue (i.e. clear follicular
space once containing gametes); N-R.T. = Non-reproductive tissue.
Numbers above each diagram represent the sample size for each
week. Note that for June 22 and September 20 in diagram labelled
“AH”, numbers include animals that could not be identified as to sex.

KENNEDY: SEASONALITY OF OYSTER LARVAL PRESENCE

27

Table 2. Settlement periodicity of Crassostrea virginica larvae throughout the species’ natural range.

Location

Study

Period

Timing of

Settlement

Peak of

Settlement

Authority

Nova Scotia,

1939-40

July-August

Varied

Medcof, 1955

Canada

Long Island

Sound,

Connecticut

1937-61

Start: July 9
to Aug. 1 1

End: Aug 26
to Oct. 17

Usually 2;
timing variable

Loosanoff, 1966

Chesapeake Bay,

Maryland

1942-53

Late May-Oct.

Varied; late June-
Sept. but usually July,
Sometimes 2 peaks

Beaven, 1955

1962-66

June-mid Sept.

Usually 2 peaks
of varying intensity

Shaw, 1969

1 977-79

Late May-
mid Sept.

Varied with year
and location

Kennedy, 1980

Virginia

1946-54

Early July-
early Oct.

Usually 2 peaks:
mid July; mid

Aug. to mid Sept.

Andrews, 1955

South Carolina

Many years

Early May-
early Oct.

Usually 2 peaks:
early June-early

July; Aug. -early Sept.

McNulty, 1953

Gulf of Mexico,

Florida

1951-61

Start: early

March to mid May
End: mid Sept,
to late Nov.

Varied

Butler, 1965

Texas

1951-52

Mid May-mid Oct.

Menzel, 1955

types, i.e., developing gametes, morphologically ripe gametes,
spawning/spent tissue, and non-reproductive tissue (mostly
interfollicular connective tissue) was recorded for three ran-
domly selected fields per tissue section. The average value
of each tissue type noted in the three fields was determined
for each oyster, resulting in four average values of tissue fre-
quency (one per tissue type) for each oyster. From these, the
percentage volume fraction of each of the four tissue types
was determined for each oyster. An average volume fraction
(%) of each tissue type was then calculated for each weekly
sample of oysters (n = 10-25).

These percentage volume fractions are presented for
males, females, and all oysters in Figure 1. For all oysters
combined there was a low incidence of non-reproductive
tissue (N-RT.) through mid August, increasing as follicles
shrank during the decline in spawning which occurred into
September. The spawning decline is marked by the drop in
percentage volume fraction of the developing and mor-
phologically ripe tissues. There was a small increase in the
percentage volume fraction of spawning/spent tissue in early
July and a larger increase in early August (this tissue type
is characterized by the space once occupied by gametes; note
that spawning activity is also indicated by an increase in the
N-R.T. component of the body due to shrinkage of follicles
upon gamete release).

Scrutiny of gametogenic events in male and female
oysters (Fig. 1) reveals three periods when the volume frac-

tions of morphologically ripe male gametes increased (early
July, late July, mid August), with four such periods occurring
for females (mid June plus the above three periods). However,
the variability in the data as reflected in the standard devia-
tions of the means (not shown in Fig. 1) was such that these
periods of increase were probably not of significance. In
general, throughout the summer until late August, gametes
were developing, ripening, and being released, with perhaps
two peaks of spawning (late June-early July and late July-early
August). Thus, larval populations were presumably always in
the water column in Broad Creek.

SETTLEMENT PERIODICITY

Settlement and metamorphosis of C. virginica usually
follows spawning in two or three weeks, depending upon water
temperatures and (probably) food quality and quantity. Thus,
settling larvae can usually be encountered throughout the
reproductive season. Table 2 lists major reports of timing of
settlement throughout the north-south range of the species.
Again, the season lengthens to the south as would be ex-
pected given the length of the spawning seasons (Table 1).
Throughout the range, it is not uncommon for there to be two
or more major settlement events (“peaks”) but this varies an-
nually, spatially, and in intensity (e.g. Beaven, 1951; Loosanoff
and Nomejko, 1956; Drinnan and Stallworthy, 1979).

This is true also within Maryland waters. Reports by

28

LARVAL ENTRAINMENT

Shaw (1969) and Kennedy (1980) covered eight years of
surveys of settlement patterns in two tributaries of the Chop-
tank River, MD (1962-1966, 1977-1979). Table 3 presents a
synopsis of the findings, demonstrating the variability of peak
periods of settlement within and between locations and years.
Italics draw attention to the period when settlement (measured
as number of spat per 100 cm2) was most intense that
season. Lack of italics where there were multiple settlement
peaks indicates a subjective judgement that there was little
difference between the intensity (spat abundance) of the
peaks.

Table 3. Variability over time and space of peak settlement periods
(measured as numbers of spat per 100 cm2) of oyster larvae in two
tributaries of the Choptank River, MD (after Shaw, 1969 and Ken-
nedy, 1980). When two or more peaks occur in a tributary, if one
peak had higher numbers of spat than the other(s), that major peak
is italisized.

Tred Avon River

Broad Creek

1962

Early August

Early and late July

1963

Early July; early August

Early July; early August

1964

Early July, mid August

Early July, early August

1965

Mid June; early July, late

Early June; late June-early

August

July, late August

1966

Late June; early July

Mid-late July

1977

Late September

Late July, late August

1978

Almost no set

Minor peak in late July

1979

Late May; mid June;
early July

Mid June; early July

In both tributaries, two or three peak periods of settle-
ment usually occurred in a season but the timing varied an-
nually and between tributaries. Settlement was always greater
in Broad Creek. This was reflected in the fact that nearly every
year the peak of settlement (spat abundance) in that tributary
exceed any peak in Tred Avon River that same year.

DISCUSSION

Examination of the literature reveals that although
broad generalizations about timing may be made, intensity
and success of spawning and settlement in C. virginica varies
with location and year in an essentially unpredictable man-
ner. Environmental factors such as temperature and the quan-
tity and quality of food supply are involved, and predation on
larvae by pelagic predators is undoubtedly also a factor. The
fact that spawning may occur is no guarantee that much spat
settlement will follow. For example, note that even though
spawning took place in Broad Creek in 1978 (Fig. 1), spat
settlement was negligible; this pattern was similar for spawn-
ing and settlement in Tred Avon River that year (Kennedy,
1 980). Definitive reasons for lack of settlement are not clear.
Low salinity has been implicated in affecting spawning and
settlement (Butler, 1949). However, although salinities were
relatively low in both tributaries in 1 978 ( = 7-9 °/00), they were
even lower in 1979 when both tributaries experienced much

greater settlement than in 1978 (Kennedy, 1980).

In conclusion, regional spawning does seem to be con-
tinuous over the summer season (Fig. 1 ; Kennedy and Krantz,
1982) as is spat settlement (Shaw, 1969; Kennedy, 1980).
Thus, given dispersion by water circulation, larval populations
can be expected to be present during the spawning period
typical for the location (Table 1). Since it is not clear what,
if any, segment of a spawning season or period of larval
availability is the most important determinant of successful
recruitment, it is difficult to predict when entrainment of oyster
larvae could occur with environmentally benign conse-
quences. Thus, for Maryland, when it is desirable to avoid
entraining larvae in a region of good spat settlement success,
such as Broad Creek, dredging should not be allowed from
June to August, unless perhaps salinities have been so low
(<5 °/00) during the previous spring as to inhibit spawning
and recruitment success (Butler, 1949).

ACKNOWLEDGMENTS

Figure 1 was prepared by Debbie Kennedy and early drafts
of the manuscript were read by V. Brock, M. Crosby and R. Newell.
Collections from Deep Neck Bar were supported by Maryland Sea
Grant Nos. 04-8-M01-73 and NA79AA-D-00058. The U.S. Govern-
ment is authorized to produce and distribute reprints for governmental
purposes notwithstanding any copyright notation that may appear
hereon. Contribution No. 1681 HPLof University of Maryland Center
for Environmental and Estuarine Studies.

LITERATURE CITED

Andrews, J. D. 1955. Setting of oysters in Virginia. Proceedings of
the National Shellfisheries Association 45:38-46.

Beaven, G. F. 1951. Recent observations on the season and pat-
tern of oyster setting in the middle Chesapeake area. Pro-
ceedings of the National Shellfisheries Association (1 950):53-59.

Beaven, G. F. 1955. Various aspects of oyster setting in Maryland.
Proceedings of the National Shellfisheries Association 45:29-37.

Butler, P. A. 1949. Gametogenesis in the oyster under conditions
of depressed salinity. Biological Bulletin 96:263-269.

Butler, P. A. 1965. Reaction of estuarine mollusks to some en-
vironmental factors. In: Biological Problems in Water Pollution,
pp. 92-104. U.S.H.E.W., Public Health Service, Cincinnati, OH.

Chesapeake Biological Laboratory. 1953. The commercial fisheries
of Maryland. Educational Series No. 30. C.B.L., Solomons
Island, MD. 45 pp.

Drinnan, R. E. and W. B. Stallworthy. 1979. Oyster larval populations
and assessment of spatfall, Bideford River, P.E.I., 1959.
Fisheries and Environment Canada, Fisheries and Marine Ser-
vice Technical Report No. 793. 16 pp.

Ingle, R. M. 1951. Spawning and setting of oysters in relation to
seasonal environmental changes. Bulletin of Marine Science
of the Gulf and Caribbean 1 : 1 1 1 -1 35.

Kennedy, A. V. and H. I. Battle. 1964. Cyclic changes in the gonad
of the American oyster, Crassostrea virginica (Gmelin). Cana-
dian Journal of Zoology 42:305-321 .

Kennedy, V. S. 1980. Comparison of recent and past patterns of
oyster settlement and seasonal fouling in Broad Creek and
Tred Avon River, Maryland. Proceedings of the National
Shellfisheries Association 70:36-46.

KENNEDY: SEASONALITY OF OYSTER LARVAL PRESENCE

29

Kennedy, V. S. and L. B. Krantz. 1982. Comparative gametogenic
and spawning patterns of the oyster Crassostrea virginica
(Gmelin) in central Chesapeake Bay. Journal of Shellfish
Research 2:133-140.

Kennedy, V. S. and L. Van Heukelem. 1985. Gametogenesis and
larval production in a population of the introduced Asiatic clam,
Corbicula sp. (Bivalvia: Corbiculidae), in Maryland. Biological
Bulletin 168:50-60.

Loosanoff, V. L. 1 965. Gonad development and discharge of spawn
in oysters of Long Island Sound. Biological Bulletin
129:546-561.

Loosanoff, V. L. 1966. Time and intensity of setting of the oyster,
Crassostrea virginica, in Long Island Sound. Biological Bulletin
130:211-227.

Loosanoff, V. L. and C. A. Nomejko. 1956. Relative intensity
of oyster setting in different years in the same areas of Long
Island Sound. Biological Bulletin 111:387-392.

Lowe, D. M., M. N. Moore and B. L. Bayne. 1982. Aspects of
gametogenesis in the marine mussel Mytilus edulis L. Jour-
nal of the Marine Biological Association of the United Kingdom
62:133-145.

McNulty, J. K. 1953. Seasonal and vertical patterns of oyster set-
ting off Wadmalaw Island, S. C. Contribution No. 15, Bears
Bluff Laboratory, Wadmalaw Island, S. C. 17 pp.

Medcof, J. C. 1955. Day and night characteristics of spatfall and
behaviour of oyster larvae. Journal of the Fisheries Research
Board of Canada 12:270-286.

Menzel, R. W. 1955. Some phases of the biology of Ostrea equestris
Say and a comparison with Crassostrea virginica (Gmelin). In-
stitute of Marine Science, University of Texas 4:70-153.

Mohr, A. W. 1982. Sediment control through dredging. In: Estuarine
Comparisons, V. S. Kennedy ed., pp. 635-646. Academic
Press, Inc., Mew York.

Sakuda, H. M. 1966. Reproductive cycle of American oyster,
Crassostrea virginica in West Loch, Pearl Harbor, Hawaii. Tran-
sactions of the American Fisheries Society 95:216-219.

Schubel, T. R. and B. C. Marcy, Jr. (eds.). 1978. Power Plant Entrain-
ment: A Biological Assessment. Academic Press, New York.
271 pp.

Shaw, W. I I 1969. Oyster setting in two adjacent tributaries of
Chesapeake Bay. Transactions of the American Fisheries
Society 98:309-314.

Truitt, R. V. 1929. Chesapeake biological inquiry, 1929. Seventh An-
nual Report of the Maryland Conservation Department, pp.
39-69. Baltimore, MD.

Truitt, R. V. 1931. Spawning and spat-fall. Ninth Annual Report of
the Conservation Department of the State of Maryland, p. 37.
Baltimore, MD.

PHYSICOCHEMICAL ALTERATIONS OF THE ENVIRONMENT ASSOCIATED
WITH HYDRAULIC CUTTERHEAD DREDGING

JOHN D. LUNZ and MARK W. LASALLE
ENVIRONMENTAL LABORATORY
U.S. ARMY ENGINEER WATERWAYS EXPERIMENT STATION
P.O. BOX 631, VICKSBURG, MISSISSIPPI 39180-0631, U.S.A.

ABSTRACT

Hydraulic cutterhead dredging operations introduce a flow field around the cutterhead and af-
fect the following physicochemical environmental alterations: levels of suspended sediments, settle-
ment of suspended sediments, dissolved oxygen demand, and chemical release from sediments. The
flow field around the cutterhead is localized to the immediate area of the suction intake, with fluid
velocities increasing with increased discharge. Suspended solids levels are generally below 200 mg//
with the highest levels near the bottom within 800 ft (243.8 m) of the dredge. Estimates of the thickness
of the sediment blanket resulting from the settlement of suspended sediments were generally less
than 4 mm. The assumption that dissolved oxygen demand is directly related to the benthic oxygen
demand of the sediment being suspended led to estimates of dissolved oxygen reductions in the water
at the dredging site; using low, moderate and high benthic oxygen demand rates and suspended solids
values. These reductions were usually less than 0.05 mg///h. The potential for release of natural and
industrially derived chemicals from sediments resuspended by the dredge is low. Apparently, this is
due to the rapid adsorption of these compounds onto clay and organic particles.

In general, physicochemical environmental alterations occurring during dredging are all related
to the suspension of sediment, while the amount of sediment in suspension is related, to a large ex-
tent, to the way the dredge itself is operated.

Environmental concerns about hydraulic dredging
operations frequently focus on associated water quality
changes. Of particular interest are changes in the levels of
suspended solids, optical turbidity, dissolved oxygen, and
potentially toxic chemical substances such as pesticides and
heavy metals. The bulk of this work has been reviewed by
Stern and Stickle (1978) and Priest (1981). Most of the studies
cited by these reviews describe water quality conditions in
the vicinity of dredges, but few have quantified these
parameters at the cutterhead itself. Sustar et al. (1976)
discussed sediment-water interactions occurring during
dredging and disposal operations and identified the primary
controlling factor as the physical properties of the sediment.
The type of dredging equipment used, operation of the equip-
ment, and site conditions were identified as additional factors.

We contend that although much remains to be learned
about dredge-induced changes in water quality, we are in a
position to report on general observations and thereby per-
mit the readers to determine the resource impact significance
of those observations. The following information was
developed for a workshop that focused on the consequences
of entrainment and mortality of oyster larvae during cutterhead

hydraulic dredging operations. Water quality conditions
around the dredge are relevant to the entrainment topic for
two reasons; first, because the physical dimensions of the
water quality field altered by the dredge may influence the
behavior and consequent survival of planktonic oyster larvae
near, although not necessarily in, the entrainment field;
second, because knowledge of water quality characteristics
affected by dredging permits a student of dredge-induced
oyster larvae mortality to decide if entrainment should be the
primary concern, or a concern shared with water quality, as
a potential cause of larval mortality.

The objective of this report is to summarize the
available information concerning several water quality
parameters in the immediate vicinity of the cutterhead of a
hydraulic dredge. In some cases, information gathered on
one parameter is used to extrapolate to associated
phenomena.

CUTTERHEAD DESCRIPTION

A complete description of a typical hydraulic dredge
and its operation is given by Barnard (1978). A typical

American Malacologicai Bulletin, Special Edition No. 3(1986):31-36

31

32

LARVAL ENTRAINMENT

• 1.1 in pipe(2.8cm)
o 1.62 in pipe (4.1 cm)
a 2.0 in pipe (5.1 cm)

1.62 in pipe
(4.1 cm)

ofront of pipe
• rear of pipe

o

a>

1/1

o

o

_J

LU

>

Fig. 1. Relationship of fluid velocity with right angle distance from a suction intake (data from Brahme, 1983 taken from Figures 18, 19, 20,
23, 24). A. Vertical pipe measurements. B. Inclined pipe measurements.

cutterhead consists of two principal systems, a cylindrical
system of rotating blades which loosens the sediment and
a hydraulic suction system which picks up and transports the
material to the dredge. Various cutterhead blade designs are
in use. Many of these designs incorporate features to increase
the efficiency of the cutterhead by directing the suspended
sediments to the suction head (Raymond, 1984). This in-
creased efficiency equates to reduced optical turbidity and
suspended sediment levels since the object of the operation
is to remove sediment.

PHYSICOCHEMICAL ALTERATIONS
AT THE CUTTERHEAD

In addition to the flow field around the suction head,
the physicochemical alterations of interest around an
operating cutterhead include: suspended sediment concen-
trations and associated optical turbidity levels; settlement of
supended sediment, a process commonly referred to as sta-
tion; dissolved oxygen concentration; and the release of
various natural or manmade chemicals associated with the
sediment. These chemicals include nutrients, heavy metals,
and pesticides. Information about these parameters must, for
the most part, be inferred from studies of dredged material
disposal. Very few direct measurements of the

physicochemical alterations affected by the operation of the
cutterhead dredge have been made compared to those af-
fected by dredged material disposal.

FLUID FIELD

Brahme (1983) described the fluid velocity fields
around experimental suction heads. Velocities were found
to increase with increased discharge through the pipe. High
velocities were generally restricted to the immediate area of
the pipe and dropped rapidly with distance, as shown in
Figure 1 . No differences were observed in a comparison be-
tween the fluid fields of vertical and inclined intakes.

SUSPENDED SOLIDS

The term turbidity, i.e., optical turbidity, is often used
in conjunction with discussions of suspended solids. Although
levels of suspended solids do affect turbidity, the two terms
are not synonymous, but are measures of two distinct
phenomena. Levels of suspended solids expressed as weight
per unit volume, typically as mg//, are the subject of the follow-
ing discussion.

Levels of suspended solids on either side and within
a 1-m radius of an operating cutterhead were measured in
the field by Yagi et al. (1977) who reported a range of 12
to 580 ppm (mg//). Values were highest on the side where
cutter blades impacted the sediment, presumably due to a

LUNZ AND LASALLE: PHYSICOCHEMICAL ALTERATIONS OF DREDGING

33

FLOOD DISTANCE, FT EBB

800 400 200 100 0 100 200 400 800

ABOVE 25 FT —11.5 mg/£ ABOVE 25 FT —37.5 mg/f

Fig. 2. Suspended sediment levels at various depths and distances from a hydraulic cutterhead dredge in the James River, Virginia (reproduced
from Raymond, 1984, Figure 3) and the Savannah River, Georgia (reproduced from Hayes, in press, Figure 2). Values for the James River
study are averages over 4 days, corrected for background suspended sediment levels. Values for the Savannah River study are combined
data for 2 days, corrected for background suspended sediment levels.

carryover of sediment particles in the water flow formed by
the rotating blades. Levels of suspended solids 2 m above
the cutterhead declined exponentially from between 5% and
74% of values near the head.

Studies by Raymond (1984) in the James River,
Virginia, and Hayes (in press) in the Savannah River, Georgia,
provide data on suspended sediments at varying distances
from an operating dredge (Fig. 2). In both studies, levels of
suspended sediments greater than 1 00 mg II above ambient
were restricted to the lower water column (8 to 10 ft above
bottom = 2.5 to 3.0 m), while levels in the upper water column
remained only slightly above ambient. In the James River
study, the ebbing tide had the effect of increasing the ver-
tical dispersion of suspended sediments.

In order to develop a model of dredge-induced turbidity
plumes, Kuo et al. (1985) measured suspended sediment
concentrations at 1 m below the surface. Values ranged be-
tween 9 and 63 mg// at distances from 67 to 375 m from the
dredge (Fig. 3). Background values of suspended solids
ranged from 9 to 35 mg//. Based on his model, sediment con-
centrations calculated for 1 ft (0.3 m) above the bottom ranged
between 2.6 and 353 mg// at distances from 22.4 to 2,240
m from the dredge (Fig. 3).

Other studies of suspended sediment levels in the
vicinity of a dredge report that high levels are generally
restricted to within 60 to 70 m horizontally from the head
with very little resuspension above the head (1 to 2 m) (Huston
and Huston, 1976; Markeyand Putnam, 1976; Sustar et al.,
1976; Barnard, 1978; Brahme, 1983; Herbich and Brahme,
1984).

SEDIMENTATION OF SUSPENDED SOLIDS

Sediment deposition was calculated in the turbidity

DISTANCE FROM SOURCE (m)

Fig. 3. Suspended sediment concentrations at various distances from
a hydraulic cutterhead dredge used in a turbidity plume model for
the Elizabeth River, Virginia (data from Kuo et at. 1985, Tables 2
and 4). Values for 1 m below the surface were estimated from field
data. Values for 1 ft (,3m) above the bottom were calculated by the
model.

plume model of Kuo et al. (1 985). Values of sediment deposi-
tion density (g/cm2) calculated for varying lateral distances
from the dredge were 0.29 (at 30.5 m), 0.28 (at 61 .0 m), 0.24
(at 152 m) and 0.15 (at 305 m). We estimated thickness of
the settled material from this data by assuming an in-situ
water content of 50% by weight and a bulk density of sedi-
ment and water equal to 2.65 g/cm3 and 1 .00 g/cm3, respec-
tively. Our estimates of sediment thickness were 4 mm at 30.5
and 61.0 m, 3 mm at 152 m and 2 mm at 305 m.

DISSOLVED OXYGEN

The effect of suspended sediment on dissolved oxygen

34

LARVAL ENTRAINMENT

(DO) has been studied largely from the standpoint of ben-
thic oxygen demand of the sediments being suspended
(Isaac, 1965; Berg, 1970; O’Neal and Sceva, 1971; Reynolds
ef a/., 1973). A few investigations have reported oxygen con-
centrations associated with dredged material disposal (U.S.
Fish and Wildlife Service, 1970; May, 1973; Slottaef a/., 1973;
Smith ef a/., 1976; Westley ef a/., 1973) and with dredging
itself (Brown and Clark, 1968). Bucket dredging activity in an
industrialized New Jersey waterway (Brown and Clark, 1968)
was shown to reduce DO levels by 16% to 83% during the
dredging period. In the case of dredged material disposal
operations, DO depressions were observed but were short-
lived and restricted to near-bottom layers. Available data on
the sediment oxygen demand of estuarine sediments, while
indicating potentially high demand levels, are complicated
by differences in measurement techniques and noncom-
parable units.

Isaac (1965) and Burdick (1976) summarized reported
benthic oxygen demand rates for a wide range of sediment
conditions. Much of this work with benthic oxygen demand
has been related to the degree of chemical contamination
or organic enrichment. Particular attention has focused on
volatile solids (VS) concentrations in sediment because of fre-
quent high correlations between benthic oxygen demand and
the percentage of volatile solids. O’Neal and Sceva (1971)
reported a significant difference in the initial oxygen demand
of sediments having relatively low volatile solids (mean =
2.9%) and those having relatively high volatile solids (mean
= 19.6%). Reynolds ef a/., (1973) reported a relationship be-
tween benthic demand (g/h-m2) and biological oxygen
demand (mg/kg).

Oxygen demand has also been shown to be a func-
tion of the amount of suspended sediment (Isaac, 1 965; Berg,
1970; Reynolds ef a/., 1973). Collett (1961, cited in Isaac,
1965) reported a substantial difference in oxygen demand
between shaken and quiescent samples from four British
rivers. Reynolds ef al. (1973) developed a predictive
mathematical model for DO demand based on the benthic
oxygen demand of the sediment. Benthic oxygen demand
(g/h-m2) was found to be directly related to sediment biological
oxygen demand (mg/kg dry wt.), temperature and degree of
resuspension.

It is clear from all these studies that oxygen demand
is directly related to a number of parameters, including the
volatile solids concentration of the sediment, temperature,
degree of sediment resuspension (concentrations of sus-
pended sediments) and duration of resuspension. Using
various levels of suspended sediments reported for hydraulic
dredging operations (Fig. 2) and estimates of low, moderate,
and high levels (5, 20, 150 /xl 02/g [sediment dry weight],
respectively) of sediment oxygen demand, we estimated
the oxygen demand for a typical dredging operation. The
DO reduction was estimated within a hypothetical closed
cylinder having a height of 2 m and a radius of 15 m, situated
around a cutterhead. The initial DO concentration in the
water within the cylinder was 5 mg//; the temperature was
25°C.

The estimated dissolved oxygen reduction (Table 1)

Table 1. Hypothetical values of dredge-induced dissolved oxygen
reduction, from 5 mg//, for combinations of suspended sediment
concentrations and benthic oxygen demand (low, 5; moderate, 20;
high, 150 /J 0/g/hr).

Suspended Sediment
Concentration
(mg//)

Benthic Oxygen
Demand

04 0/g/hr)

Dissolved Oxygen
Reduction
(mg///hr)

100

low

0.01

100

moderate

0.01

100

high

0.03

200

low

0.01

200

moderate

0.01

200

high

0.05

300

low

0.01

300

moderate

0.01

300

high

0.07

500

low

0.01

500

moderate

0.02

500

high

0.11

1500

high

3.00

is generally small (<0.05 mg//) for all combinations of sus-
pended sediment and oxygen demand levels. By these com-
putations, a considerable reduction of DO (3.0 mg//) would
occur under combined conditions of extremely high suspend-
ed solids and high oxygen demand. Dissolved oxygen depres-
sions observed during dredged material disposal operations
are higher ( > 1 .0 mg//) than those we estimated at the dredg-
ing location; however, the levels of suspended sediment are
much greater during dredged material disposal. The DO
depressions at dredged material disposal sites are reported
to be short lived, on the order of hours (U.S. Fish and Wildlife,
1970; May, 1973; Westly et al., 1973; Barnard, 1978).

CHEMICAL RELEASE FROM SEDIMENTS

The release of naturally occurring chemical
substances (nutrients, sulfides, iron, and other metals in
naturally occurring concentrations) and industrially derived
chemical substances (potentially toxic metals in higher than
natural concentrations, organohalogen compounds, and
pesticides) by the suspension of sediments during dredging
is a concern, particularly in highly contaminated sites. The fate
of these compounds when sediments are suspended has
received much attention and is thoroughly reviewed by Stern
and Stickle (1978). Information from laboratory studies sug-
gests that metals from bottom sediments, released when
these sediments are suspended, are adsorbed onto clay and
organic particles in the water column. Estimates of the per-
cent of various metals adsorbed in this way include 69.8%
for copper, 97.4% for mercury, and 50% and 75% for mer-
curic nitrate and mercury chloride. In field situations, the
levels of mercury in the water column were reported to be
negligible or very slight (Stern and Stickle, 1978).

Windom (1972) suggested a general explanation of ad-
sorption involving iron found in the sediment. Reduced iron,
once oxidized during resuspension, actively scavenges
metals and other compounds, thereby removing them from

LUNZ AND LASALLE: PHYSICOCHEMICAL ALTERATIONS OF DREDGING

35

the water column. These compounds accumulate in the set-
tling sediments where they are reduced upon return to anoxic
conditions. For the most part, therefore, these compounds
are generally not available in the water column except as part
of an iron complex and are retained in the sediment by reset-
tlement of these complexes. May (1974) made similar obser-
vations during dredged material disposal into containment
basins and suggested three possible scenarios to explain
them including: a) the processes acting under conditions of
dredged material disposal are either too slow or are too quick-
ly reversible to allow sediment constituents to become dis-
solved: b) the materials are too strongly adsorbed to either
inorganic or organic matter so that reducing conditions have
little effect on them; c) the materials in question are largely
in refractory forms (organic and inorganic complexes) and
are not readily soluble.

The degree of release of chlorinated hydrocarbons dur-
ing dredging per se is unknown (Stern and Stickle, 1978;
Priest, 1981). The behavior of these compounds in sediment-
water systems associated with dredged material disposal
alternatives was reviewed by Gambrell et al. (1978) who
reported that chlorinated hydrocarbons are strongly bound to
the solid phase in typical soil and sediment-water systems.
They state, “unless a contaminated sediment is coarse-
textured with low organic matter content, dissolved com-
pounds will exist at extremely low levels such that this form
will not be an acute environmental threat”. Their review fur-
ther points out that an exception may be where a very high
suspended solids to water ratio exists and they describe
results of laboratory elutriate tests reported by Lee et al.
(1975) in which a 20% suspended solids system resulted in
release to soluble forms while a 5% suspended solids system
showed less release or no release. Laboratory investigations
conducted by Chen et al. (1976) suggest that very fine silts
and clays and the quantities of humic and fulvic acids in
marine sediments are important factors in controlling the ad-
sorption capacity of chlorinated hydrocarbons in marine
sediments. These laboratory observations led to a conclu-
sion that chlorinated hydrocarbons will not be released to
solution in detectable quantities under normal conditions.
However, these compounds undergo similar association with
organic detritus and inorganic sediment particles and are
capable of being transferred through the food chain via in-
gestion by organisms (Chen et al., 1976; Burks and Engler,
1978; Rubinstein et al., 1984). Where substantial suspended
solids are present, the oil and grease content of the sus-
pended particulates is reported to be more important in
regulating total levels of suspended chlorinated hydrocarbons
than actual levels of suspended solids (Burks and Engler,
1978). Lee et al. (1975) suggested the simultaneous occur-
rence of both oil and grease and chlorinated hydrocarbons
may enhance bioavailabiiity.

Quantitative information about the release of nutrients
during hydraulic cutterhead dredging operations is not
available. Qualitative information on the behavior of nitrogen
and phosphorus compounds during dredged material
disposal in open water can be viewed as applicable as long
as the differences in dredging and dredged material disposal

operations are recognized in terms of the concentrations and
volumes of sediment that they suspend. Stern and Stickle
(1978) concluded that turbidity and suspended material can
play both a beneficial and detrimental role in aquatic en-
vironments. They state that suspended material sorbs and
removes contaminants from the water column and stimulates
photosynthesis through the introduction of inorganic nutrients.
They further conclude that nutrients may stimulate excessive
biological growth and that turbidity might reduce photosyn-
thetic activities because of its interference with light penetra-
tion. There is a consensus among reviewers of nutrient
release during dredged material disposal operations that am-
monia toxicity or an increase in algae productivity should not
be problems in most waters where substantial dilution and
mixing occur.

INFLUENCE OF DREDGE OPERATING
CONDITIONS ON PHYSICOCHEMICAL
ALTERATIONS

Several features of cutterhead dredge operations can
influence the physicochemical alterations previously dis-
cussed. These include the rate of suction, cutterhead rota-
tional speed, size of the cutterhead, depth of the cut, and
the speed with which the cutterhead is swung back and forth
across the area being dredged. All of these phenomena have
been reported to influence levels of suspended solids adja-
cent to the cutterhead (Vagi ef al., 1975; Huston and Huston,
1976; Barnard, 1978; Brahme, 1983; Herbich and Brahme,
1984; Hayes et al., 1984) and the efficiency of operation of
hydraulic dredges (Raymond, 1984). It is apparent that
specifications for operating a cutterhead dredge to achieve
optimum efficiency are compatible with specifications that
would be intended to minimize potentially adverse
physicochemical alterations.

ACKNOWLEDGMENTS

We thank Douglas Gunnison, (WES) for information on the
oxygen demand of estuarine sediments.

LITERATURE CITED

Barnard, W. D. 1978. Prediction and control of dredged material
dispersion around dredging and open-water pipeline disposal
operations. Technical Report DS-78-13. U.S. Army Engineer
Waterways Experiment Station, Vicksburg, Mississippi. 112 pp.
Berg, R. H. 1970. The oxygen uptake demand of resuspended bot-
tom sediments. Water Pollution Control Research Series No.
1 6070-DCD-09/70. U.S. Environmental Protection Agency,
Washington, DC. 34 pp.

Brahme, S. B. 1983. Environmental aspects of suction cutterheads.

Ph.D. Dissertation, Texas A&M University. 166 pp.

Brown, C. L. and R. Clark. 1968. Observations on dredging and
dissolved oxygen in a tidal waterway. Water Resources
Research 4:1381-1384.

Burdick, J. C., III. 1976. Analysis of oxygen demand of sediments.
In: Proceedings of the Specialty Conference on Dredging and
its Environmental Effects , P. A. Krenkel, J. Harrison and J.

36

LARVAL ENTRAINMENT

C. Burdick III, eds. pp. 319-352. American Society of Civil
Engineers.

Burks, S. A. and R. M. Engler. 1978. Water quality impacts of aquatic
dredged material disposal (laboratory investigations). Syn-
thesis report. Technical Report DS-78-4. U.S. Army Engineer
Waterways Experiment Station, Vicksburg, Mississippi. 35 pp.

Chen, K. Y., S. K. Gupta, A. Z. Sycip, J. C. S. Lu, M. Knezevic and
Won Wook Choi. 1976. Research study on the effect of disper-
sion, settling, and resedimentation on migration of chemical
constituents during open-water disposal of dredged materials.
Contract Report D-76-1 . U.S. Army Engineer Waterways Ex-
periment Station, Vicksburg, Mississippi. 221 pp.

Collett, W. F. 1961. A preliminary investigation of the pollution of
the upper Forth estuary. In: Journal and Proceedings of the
Institute of Sewage Purification, Part 5:418-433.

Gambrell, R. P., R. A. Khalid and W. H. Patrick, Jr. 1978. Disposal
alternatives for contaminated dredged material as a manage-
ment tool to minimize adverse environmental effects.
Technical Report DS-78-8. U.S. Army Engineer Waterways
Experiment Station, Vicksburg, Mississippi. 148 pp.

Hayes, D. F. In press. Guide to selecting a dredge for minimum
resuspension of sediment. Environmental Effects of Dredg-
ing Technical Notes, EEDP-09-1 . U.S. Army Engineer Water-
ways Experiment Station, Vicksburg, Mississippi. 9 pp.

Hayes, D. F., G. L. Raymond and T. N. McLellan. 1984. Sediment
resuspension from dredging activities. In: Dredging and
Dredged Material Disposal, R. L. Montgomery and J. W. Leach,
eds. pp. 72-82. Proceedings of the Conference Dredging '84.,
Clearwater Beach, Florida, November 14-16, 1984.

Herbich, J. B. and S. B. Brahme. 1984. Turbidity generated by a
model cutterhead dredge. In: Dredging and Dredged Material
Disposal, R. L. Montgomery and J. W. Leach, eds. pp. 47-56.
Proceedings of the Conference Dredging ’84., Clearwater
Beach, Florida, November 14-16, 1984.

Huston, J. W. and W. C. Huston. 1976. Techniques for reducing tur-
bidity associated with present dredging procedures and opera-
tions. Contract Report D-76-4. U.S. Army Engineer Waterways
Experiment Station, Vicksburg, Mississippi. 78 pp.

Isaac, P. C. G. 1 965. The contribution of bottom muds to the deple-
tion of oxygen in rivers and suggested standards for sus-
pended solids. In: Biological Problems in Water Pollution, U.S.
Public Services Publication No. 999-WP-25. pp. 346-354.

Kuo, A. Y., C. S. Welch and R. J. Lukens. 1985. Dredge induced
turbidity plume model. Journal of Waterway, Port, Coastal and
Ocean Engineering. 111:476-494.

Lee, G. F., M. D. Piwoni, J. M. Lopez, G. M. Mariani, J. S. Richard-
son, D. H. Homer and F. Saleh. 1975. Research study for the
development of dredged material disposal criteria. Contract
Report D-75-4. U.S. Army Engineer Waterways Experiment
Station, Vicksburg, Mississippi. 337 pp.

Markey, J. W. and H. D. Putnam. 1976. A study of the effects of
maintenance dredging on selected ecological parameters in
the Gulfport Ship Channel, Gulfport, Mississippi. In: Pro-
ceedings of the Specialty Conference on Dredging and its En-
vironmental Effects, P. A. Krenkel, J. Harrison and J. C. Bur-
dick III, eds. pp. 821-832. American Society of Civil Engi-
neers.

May, E. B. 1973. Environmental effects of hydraulic dredging in
estuaries. Alabama Marine Resources Bulletin 9:1-85.

May, E. B. 1 974. Effects on water quality when dredging a polluted
harbor using confined spoil disposal. Alabama Marine

Resources Bulletin 10:1-8.

O’Neal, G. and J. Sceva. 1971. The effects of dredging on water
quality in the northwest. Report No. PB 228 533. Environmental
Protection Agency, Region X, Seattle, Washington. 158 pp.

Priest, W. I. 1981. The effects of dredging impacts on water quality
and estuarine organisms: a literature review. Virginia Institute
of Marine Science. Special Report in Applied Marine Science
and Ocean Engineering. 247:240-266.

Raymond, G. L. 1 984. Techniques to reduce the sediment resuspen-
sion caused by dredging. Miscellaneous Paper HL-84-3. U.S.
Army Engineer Waterways Experiment Station, Vicksburg,
Mississippi. 33 pp.

Reynolds, T. D., R. W. Hann, Jr. and W. F. Priebe. 1973. Benthic
oxygen demands of Houston Ship Channel sediments. Texas
A&M University. TAMU-SG-73-204. 58 pp.

Rubinstein, N. I., W. T. Gilliam and N. R. Gregory. 1984. Dietary
accumulation of PCBs from a contaminated sediment source
by a demersal fish species (Leiostomus xanthurus). Technical
Report D-84-6. U.S. Army Engineer Waterways Experiment
Station, Vicksburg, Mississippi. 25 pp.

Slotta, L. S., C. K. Sollitt, D. A. Bella, D. R. Hancock, J. E. MaCauly
and R. Parr. 1973. Effects of hopper dredging and in chan-
nel spoiling in Coos Bay, Oregon. School of Engineering and
Oceanography, Oregon State University, Corvallis, Oregon.
133 pp.

Smith, J. M., J. B. Phipps, E. D. Schermer and D. F. Samuelson.
1976. Impact of dredging on water quality in Grays Harbor,
Washington. In: Proceedings of the Specialty Conference on
Dredging and its Environmental Effects, P. A. Krenkel, J. Har-
rison and J. C. Burdick III, eds. pp. 512-528. American Society
of Civil Engineers.

Stern, E. M. and W. B. Stickle. 1978. Effects of turbidity and sus-
pended material in aquatic environments: literature review.
Technical Report D-78-21. U.S. Army Engineer Waterways Ex-
periment Station, Vicksburg, Mississippi. 117 pp.

Sustar, J. F., T. H. Wakeman and R. M. Ecker. 1976. Sediment-water
interaction during dredging operations. In: Proceedings of the
Specialty Conference on Dredging and its Environmental Ef-
fects, P. A. Krenkel, J. Harrison and J. C. Burdick III, eds.
pp. 736-767. American Society of Civil Engineers.

U.S. Fish and Wildlife Service. 1970. Effects on fish resources of
dredging and spoil disposal in San Francisco and San Pablo
Bays, California. Unnumbered Special Report, November
1970. Washington, DC. 36 pp.

Westley, R. E., E. Finn, M. I. Carr, M. A. Tarr, A. J. Schotz, L. Good-
win, R. W. Sternberg and E. E. Collias. 1973. Evaluation of
effects of channel maintenance dredging and disposal on the
marine environment in Southern Puget Sound, Washington.
Washington Department of Fisheries Management and
Research Division, Olympia, Washington. 308 pp.

Windom, H. L. 1972. Environmental aspects of dredging in estuaries.
J. W. Harbor Coast. Eng. Div. ASCE 98:475-487.

Yagi, T., T. Koiwa and S. Miyazaki. 1977. Turbidity caused by dredg-
ing. In: Proceedings of WODCON VII: Dredging, Environ-
mental Effects and Technology, pp. 1079-1109.

Yagi, T., S. Miyazaki, Y. Okayama, A. Koreishi, Y. Sato, M. Saito,
Y. Nakazono, K. Masuda, S. Kono, Y. Shibuya, K. Kikuchi and
T. Kikuya. 1975. Influence of operating condition against
dredging capacity and turdidity. Technical Note No. 228. Port
and Harbor Research Institute, Ministry of Transport, Japan.
41 pp.

PREDICTION OF FLOW FIELDS NEAR THE SUCTION
OF A CUTTERHEAD DREDGE

E. C. McNAIR, JR. and GLYNN E. BANKS
U. S ARMY ENGINEER WATERWAYS EXPERIMENT STATION
VICKSBURG, MISSISSIPPI 39180-0031, U. S. A,

ABSTRACT

As dredging takes place in a waterway, a flow field is established in the vicinity of the dredge
suction. Particles or organisms that are near the dredge suction are subject to being drawn into the
dredge suction and passed through the dredge. Research has been performed that defines the flow
field surrounding the suction of a cutterhead dredge. This paper contains brief descriptions of some
research efforts into the strength of flow fields near a dredge suction and presents sample calcula-
tions of flow field velocities in the vicinity of a dredge suction pipe for a typical dredging application.

Establishing and maintaining navigable waterways is
a necessary process in a viable, strong, and healthy economy.
The principle method for maintaining and establishing these
waterways in the United States is through dredging. Dredg-
ing is the art of using mechanical and hydraulic means for
moving bottom sediments from a location where they are not
wanted to a location that is specially designed for their place-
ment and containment.

The art of dredging has been vastly improved in the
last decade in terms of effectiveness and efficiency and in
consideration of environmental factors. In spite of these im-
provements, however, some naturally occurring phenomea
are subject to disruption by the dredging process. One of
these natural phenomena, the spawning and larval growth
of oysters, is of particular concern since dredging sometimes
occurs during this event. While there is no conclusive data
relating to the impact of dredging on oyster larvae, it is of
interest to determine some characteristics of the dredge to
serve as a predictive tool in judging any impact that can occur.

The objective of this paper is to describe research that
has been performed that defines the flow field surrounding
the suction of a cutterhead dredge. There is no attempt to
describe the response of any organism that could be present
in the flow field. The analytical methods described, however,
should provide a means for predicting the velocities, and
therefore the forces acting on a particle in the flow field.

THE DREDGE

A hydraulic dredge is a floating device that removes
bottom sediments by entraining them in induced water flow
and transporting them in closed pipes to a designated

disposal area. The flow of fluid or mixture of fluid and sedi-
ment into the dredge pipe is induced by creating a lower-than-
ambient pressure condition in the dredge suction pipe. This
lower-than-ambient pressure is generated mechanically by
a solids-handling pump located on the dredge. This solids-
handling pump is usually a centrifugal type pump, a design
that has been in use for decades.

The dredged mixture enters the dredge through the
suction pipe. The entrance to the suction pipe can be en-
hanced in a variety of ways and the various ways of treating
the suction provides a description of the dredge. For instance,
if the suction pipe is not modified in any way, the dredge is
known as a “plain suction” dredge. If a device is added to
the suction pipe to assist in loosening the bottom sediments
to make them easier to ingest into the dredge suction pipe,
the dredge is usually known by the device that is added. The
most common types of additions to dredge suctions are cut-
terheads, bucketwheels (a special type of cutterhead), and
dustpans.

The cutterhead dredge (Fig. 1) is the prominent type
of dredge used in the United States for construction and
maintenance dredging activities in inland and coastal waters.
A typical dredge consists of a rigid hull equipped with a diesel
engine driving a centrifugal pump with either a hydraulic or
electric powered cuttershaft. Electric or hydraulic winches are
used to raise and lower the spuds around which the dredge
pivots. The dredge operates by clearing an arc of cut with
a rotating cutter at the end of its ladder and pumping this
material through the discharge line supported by pontoons.
Once an arc of cut ranging from a couple of feet for small
dredges to ten-fifteen feet for very large dredges has been
completed, the dredge advances down the channel by drop-

American Malacological Bulletin, Special Edition No. 3(1 986): 37-40

37

38

LARVAL ENTRAINMENT

Fig. 1. The cutterhead dredge (From Huston and Huston, 1976).

ping its walking spud while the dredge is stationary near the
corner of the channel, raising its digging spud and swinging
toward the opposite corner of the channel for several degrees,
then dropping its digging spud, where the dredging cycle of
clearing another arc of cut begins.

The traditional basket cutter (Fig. 2) is the most com-
mon type of cutter used on dredges for maintenance-type
dredging operations. It consists of a multibladed excavator
which rotates around a longitudinally mounted shaft. These
cutters vary in shape, number of blades, type of cutting edges,
etc., based on the type of material being loosened,
horsepower available, and types of trash or debris lodged in
the sediments. Since the cutter protects the suction pipe to
some extent, it acts as a device to prevent excessive stresses
on the suction pipe. In order to accomplish this function, it

Fig. 2. The basket cutter (From Huston and Huston, 1976).

must displace the bottom material and bring it within the in-
fluence of the suction inlet. To be effective, this suction inlet
should be essentially in contact with the material or even
buried.

As the dredge maneuvers in the dredging project, the
suction is brought into contact with bottom sediments. The
cutterhead, which usually has a serrated or toothed edge,
contacts the bottom and loosens sediments so that the in-
duced flow can erode the sediments more easily and carry
them into the suction pipe. In this manner, the cutterhead
dredge transfers bottom sediments from one location, where
they interfere with navigation or other activities, to another
location that is designed and designated for their
containment.

A fact of dredge operation is that any and all contents
of the flow field surrounding the dredge suction are subject
to being drawn into the dredge and transported through the
dredge system. The desirable contents of this zone of in-
fluence are bottom sediments and a sufficient amount of pore
water or ambient water to assure proper transport of
sediments through the dredge pipes. However, if other ob-
jects, such as oyster larvae, are present in the zone of in-
fluence, they can also be drawn into the dredge system.

Apgar and Basco (1973) compared experimental
results of a plain-ended suction pipe with predictions of a
potential flow model. Their results agreed relatiavely well for
some cases, but limitations to their experimental apparatus
affected their results and they were not able to improve their
model. In general, they found that their potential flow model
and their experiments showed that the velocity distributions
surrounding a plain-ended suction pipe were such that
velocities in the fluid decreased rapidly with distance from
the suction inlet.

Slotta (1968, 1974) has studied the mechanics of fluids
entering a dredge suction in an effort to develop design

MCNAIR AND BANKS: PREDICTION OF FLOW FIELDS

39

criteria for dredge cutterheads. His findings indicated that the
flow field probably followed Reynold’s criterion and was in-
fluenced by fluid viscosity.

Gladigau (1975) extended the studies of Slotta and in-
dicated that the Reynold’s criteria were not dominant. He sug-
gested that a Froude-type relationship exists for entrainment
of sand particles.

Brahme (1983) carried out a comprehensive ex-
perimental program to describe the flow field in the vicinity
of a dredge suction. He extended the work of others and
showed that the velocity of the fluid approaching the dredge
suction intake could be predicted if certain criteria were
satisfied.

Brahme (1983) found that he could approximate the
flow fields of an angled dredge suction pipe by using a ver-
tical pipe (Fig. 3) with only minor anomalies. His experiments
made use of three different pipe diameters, and several flow
rates and suction inlet positions with reference to the bottom.
In none of his experiments did he simulate a bed made of
erodable material, nor did he allow the pipe to contact the
fixed bed of his experimental flume. His experiments are
therefore a simplified representation of dredging but they are
a “worst case” example for oyster larvae entrainment
because the flow field is probably strongest with the dredge
suction in a position away from the bottom.

His experiments consisted of measuring velocities in
the fluid surrounding the pipe as a constant volume of fluid
was pumped from the flume through the pipe. After measur-
ing the velocity field in the fluid for a specified test condition

Fig. 3. Flow fields near a vertical suction pipe (From Brahme, 1 983).

Fig. 4. Plot of Q/(r2V) for different values of x/h and r/H (From Brahme,
1983).

with the pipe at a given distance from the floor and a specified
flow rate pumped through the pipe, one of the test parameters
was changed and the velocity structure was again measured.
When all test parameters had been evaluated, a pipe of a
different diameter was installed in the test apparatus and the
test sequence was repeated for all parameters.

Analysis of experimental data was undertaken to deter-
mine whether any relation existed between the various
parameters that influenced the velocity field in the vicinity of
the intake. Values of the dimensionless number “Q/(r2V)”
were computed for different values of x/h and r/h, where “r”
is the radical distance from the center of the pipe, “H” is the
depth of water in the tank, “x” is the vertical distance above
the bottom, “h” is the distance of intake of pipe above the
bottom, “Q” is the discharge in the pipe, and “V” is the
velocity of the fluid in the vicinity of the suction pipe (Fig. 4).
Detailed analysis of experimentally obtained velocity fields
revealed that values of Q/(r2V) for different velocity fields were
similar. Further study of laboratory results based on com-
paring equal values of Q/(r2V) led Brahme to the conclusion
that the magnitude of velocity in the vicinity of an intake can
be reasonably well computed for any condition of discharge
(Q), diameter (d), distance the intake is above the bottom (h),
and depth of water (H), provided that the values of h/H lie
between 0.1 and 0.225 and the values of d/H lie between 0.03
and 0.055.

SAMPLE CALCULATION

A calculation using typical values for dredge

40

LARVAL ENTRAINMENT

parameters will illustrate the method for determining velocities
in the fluid near the dredge suction. The sample calculations
are for a dredge with a 304.8 millimeter diameter suction
operating in water depths of 6.1 meters with the suction 0.61
meters from the bottom. The velocities are to be determined
at a level in the fluid 0.61 meters from the bottom at radial
locations 0.76 meters from the suction pipe and 1 .5 meters
from the suction pipe. These locations are marked “A” and
“B” respectively on Figure 4.

For Location A:

d = diameter of suction pipe = 0.3048 m
H = depth of water = 6.1 m
r = radial distance from center of pipe = 0.76 m
h = distance of intake of pipe above bottom =
0.61 m

O = discharge in suction pipe = 0.231 m3/sec
x = vertical distance from bottom = 0.61 m
x/h = 0.61/0.61 = 1.0
h/H = 0.61/6.1 = 0.1
d/H = 0.3048/6.1 = 0.05
r/H = 0.76/6.1 = 0.125
From Figure 4, Q/(r2V) = 10

Therefore,

VA = Q/(r2 x 10) = 0.231/(0.762 x 10) = 0.040 m/sec
For Location B:

d = diameter of suction pipe = 0.3048 m
H = depth of water = 6.1 m
r = radial distance from center of pipe = 1.52 m
h = distance of intake of pipe above bottom =
0.61 m

Q = discharge in suction pipe = 0.231 m3/sec
x = vertical distance from bottom = 0.61 m
x/h = 0.61/0.61 = 1.0
h/H = 0.61/6.1 = 0.1
d/H = 0.3048/6.1 = 0.05
r/H = 1.52/6.1 = 0.25
From Figure 4, Q/(r2V) = 5.25

Therefore,

VB = Q/(r2 x 5.25) = 0.231/(1. 522 x 5.25) =

0.019 m/sec

These sample calculations demonstrate the applica-
tion of the technology developed by Brahme to describe the
velocity of the fluid in the vicinity of a dredge suction. Much
additional research is needed in order to completely describe
the complex flow patterns that occur with a plain suction and
the even more complex situation with a cutterhead attached
to the dredge suction. However, for the present, the work
described above serves to give an order-of-magnitude predic-
tion of the strength of the flow field.

ACKNOWLEDGMENT

Permission of the Chief of Engineers to prepare and publish
this paper is acknowledged.

LITERATURE CITED

Brahme, S. B. 1 983. Environmental Aspects of Cutterhead Dredges.
A Dissertation submitted to the Graduate College of Texas
A&M University, College Station, Texas. 166 pp.

Apgar, W. J. and D. R. Bascoe. 1973. An Experimental and
Theoretical Study of the Flow Field Surrounding A Suction
Pipe Inlet. Texas A&M Sea Grant College Publication TAMU-
SG-74-203. 71 pp.

Slotta, Larry S. 1968. Flow visualization techniques used in dredge
cutterhead evaluation. 1968 Proceedings of WODCON, Rot-
terdam, The Netherlands, pp. 56-77.

Slotta, Larry S. 1974. Cutterhead research and standardization. Pro-
ceedings of the Fifth World Dredging Conference WODCON V,
Hamburg, West Germany, pp. 437-474.

Gladigau, L. N. 1975. Interactions between sand and water. Pro-
ceedings of the Sixth World Dredging Conference, WODCON
VI, Taipei, Taiwan, pp. 261-294.

Huston, John. 1967. Dredging Fundamentals. Journal of the Water-
ways and Harbors Division. Proceedings of the American Socie-
ty of Civil Engineers, 93 (WW3):45-69.

Huston, John, and William C. Huston. 1976. Techniques for reduc-
ing turbidity associated with present dredging procedures and
operations. Contract Report D-76-4. U. S. Army Engineer
Waterways Experiment Station. Vicksburg, MS. 33. pp.

INFLUENCE OF SUSPENDED PARTICLES ON BIOLOGY
OF OYSTER LARVAE IN ESTUARIES

MELBOURNE R CARRiKER
COLLEGE OF MARINE STUDIES
UNIVERSITY OF DELAWARE
LEWES, DELAWARE 19958, U.S.A.

ABSTRACT

Oysters of the genus Crassostrea have thrived in estuaries and semi-enclosed embayments
over geologic time although exposed to heavy suspensions of a wide variety of inorganic and organic
particles. The impact of these particles on the ecology, behavior, and physiology of oyster larvae dur-
ing the approximately 14 days of their planktonic existence and settlement has been explored only
in a preliminary way. This paper reviews the effects of the range of sizes, qualities, and concentra-
tions of natural estuarine and coastal suspended materials on the planktonic stages of Crassostrea
virginica (Gmelin). Stages considered include spermatozoa, ova, embryos in egg membranes, free
swimming trocbophores, veiigers (straight-hinged to mature and eyed umbonai stages), and pediveligers.
Physiological functions considered include ciliary feeding, respiration, locomotion (velar swimming
and pedal crawling), formation and growth of shell, velar formation and loss, pedal development and
resorption, and byssus formation and shell affixation to hard substrata. The review clearly shows that
high concentrations of suspended materials can be harmful to all oyster planktonic stages, some kinds
and sizes of particles more than others. The dividing range of concentrations of particles between
beneficial and harmful, however, has been only roughly indicated by preliminary research. Concen-
trations of suspended materials below approximately 0.18 g// for embryos in the egg membrane, and
below 0.5 gll for veiigers, can be beneficial; above these levels suspended particles are increasingly
harmful as concentrations increase, particularly to the degree toxic substances are adsorbed on
suspended dispersed particles or incorporated in suspended aggregrates.

Oysters of the genus Crassostrea thrive in estuaries and semi-
enciosed embayments (Gaitsoff, 1964; Kennedy and Breisch,
1981) where heavy loads of suspended particles are a domi-
nant ecological factor (Riley, 1970; Moore, 1977; Biggs, 1978).
These materials (often called silt for the sake of brevity) are
a complex mixture of particles of different size, shape, specific
gravity, and mineraiogieal and organic composition. Units con-
sist of both dispersed particles and aggregations, the majority
being in the silt and clay category and generally less than
15 /xm in size. Small dispersed particles are carried in relative-
ly permanent suspension, whereas larger dispersed particles
and floccular aggregates settle out in quiet areas. Organic-
inorganic complexes (detritus), fecal pellets, and dissolved
organics also constitute a substantial fraction of materials in
estuaries (Nelson, 1962; Manahan and Crisp, 1982;
Stephens, 1983). The influence of suspended particles on
the ecology, behavior, and physiology of oyster larvae dur-
ing the approximately 14 days of their planktonic existence
and settlement has been explored only in a preliminary way.

It is obvious, however, that having evolved in coastal
habitats over geologic time (Stenzel, 1971), oysters and their

larval stages have adapted to conspicuously high concen-
trations of suspended materials. So it is thus not unexpected
that “normal” estuarine concentrations of suspended par-
ticles could be beneficial to them. Ali (1982), for example,
demonstrated in laboratory experiments using suspended
particles (oxidized with 30% hydrogen peroxide) collected in
a local estuary (Broad Creek) that growth of young post-set
oysters increased with increase of suspended sediment con-
centrations up to 25 mg// when given with a standard food
ration. Concentrations up to 1 50 mg// had no adverse effect.
At the time of the experiments concentration of suspended
sediment in the local estuary averaged 68 mg//.

The present literature synthesis concerns the higher
than “normal” concentrations, periods of exposure, and
range of size and quality of particles at which natural estuarine
and coastal suspended materials become injurious to
planktonic stages of Crassostrea virginica (Gmelin).

Planktonic stages to be considered include sper-
matozoa, ova, embryos in egg membranes, free swimming
trochophores, veiigers (straight-hinged to mature and eyed
umbonai stages), and pediveligers. Physiological functions

American Malacological Bulletin, Special Edition No. 3(1986):41-49

41

42

LARVAL ENTRAINMENT

that could be affected deleteriousSy by excessive concentra-
tions of suspended particles include ciliary feeding, respira-
tion, locomotion (velar swimming and pedal crawling), forma-
tion and growth of the shell, velar formation and loss, pedal
development and resorption, and byssus formation and shell
affixation to hard surfaces.

In the region from New Jersey to southeastern Canada
veligers range in length from about 60 /x m (straight-hinged
larvae, anteroposterior dimension) to as large as 400 fim (eyed
pediveligers ready to set) (Carriker, 1951). Maximal length
of the shell of ready to set pediveligers increases gradually
northward along the coast, smallest pediveligers occurring
in New Jersey and largest in Canada (Nelson, 1921; Carriker,
1951 ; Galtsoff, 1964). Straight-hinged larvae are always more
abundant than older stages, losses from predation and other
causes accounting for the heavy diminution of larval popula-
tions with time. By the time pediveligers are ready to set, mor-
tality can be as high, or higher, than 99%, though it has not
been possible to determine this loss accurately In the field.

EARLY NON-SWIMMING PLANKTONIC STAGE

Spawning oysters discharge ova and spermation in a
more or less steady stream that spreads out and disperses
several centimeters from each bivalve. Release of gametes
can last from a few minutes to an hour or more. Toward the
end of the reproductive season, shedding of sperm can con-
tinue for several hours until males are completely spent. In
the water over large populations of spawning oysters released
gametes form “clouds” of cells that take the shape of long
lanes as the tidal water flows back and forth over the bottom
(Nelson, 1921 ; Galtsoff, 1964). Mass spawning is induced by
stimulation of females by a hormone-like substance, diantlin,
carried on spermatozoa (Nelson and Allison, 1940). The ef-
fect of suspended particles on diantlin and its stimulatory fun-
tion has not been determined.

Embryos develop rapidly within the vitelline (egg) mem-
brane to the characteristically ciliated trochophore stage. The
trochophore emerges from the egg membrane in 6 to 8 hours
to become a free-swimming zooplankter. With ongoing tidal
mixing the original cloud of cells and embryos disperses both
vertically and horizontally, though the resulting swarm of
veligers tends to maintain its identity for some time in the
larger mass of water (Galtsoff, 1964; Andrews, 1979). Ova
and spermatozoa secrete fertilization substances (gamones)
that promote fertilization (Sastry, 1979). The effect of
suspended particles on this process is unknown, though
gamones could be adsorbed on clays or other particles,
preventing normal fertilization. The matter is important and
needs clarification.

It would seem that, sheltered within the egg mem-
brane, the developing blastula, gastrula, and early
trochophore would be unaffected by suspended particles. On
the contrary, Davis (Loosanoff, 1962) and Davis and Hidu
(1969b) found in laboratory experiments that concentrations
of silt of 0.25 g // resulted in 27% mortality of embryos within
egg membranes; at 0.5 gll 69% died, and in 1.0 to 2.0 gll
mortality ranged from 97 to 100%.

Suspensions of kaolin and Fuller’s earth were less
harmful than silt. For example, in a concentration of 1.0 gll
of these two materials almost all embryos developed to the
straight hinge stage, and some developed normally even in
concentrations of 4.0 gll (26% in Fuller’s earth and 76% in
kaolin), while in a concentration of only 1.0 g/7 of silt prac-
tically none (3%) of the embryos reached the straight-hinged
stage (Davis and Hidu, 1969b).

Although as little as 0.168 gll of silt had a noticeable
deleterious effect on the percentage of embryos developing
to the straight-hinged stage, the percentage of embryos
developing normally in concentrations of silicon dioxide (sand)
up to 4 gll, regardless of particle size, was not affected. Davis
and Hidu (1969b) concluded tentatively that larger particles
present in silt, and to a lesser extent in kaolin and Fuller’s
earth, were primarily responsible for deaths of embryos in
egg membranes.

These preliminary results point to the importance of
determining the effect of the various components of suspend-
ed materials from different estuaries and coastal regions on
oyster embryos and larvae prior to disturbance of the bot-
tom by dredging or other mechanical means.

FREE-SWIMMING PLANKTONIC STAGES

The naked, fragile, free-swimming trochophore, un-
protected by the egg membrane, is undoubtedly a highly
vulnerable larva, not only to predators but also to stressful
conditions in the physical and chemical environment. The lar-
va remains in the trochophore stage between 24 and 48 hours
(Galtsoff, 1964), Since it begins to feed at this stage, the
presence in its immediate surroundings of a dense suspen-
sion of sedimentary particles could clog the mouth and
esophagus and entangle the ciliated ring (prototroch)— unless
the larva can keep the mouth clear and selectively ingest
nutritious particles from the suspension, which is question-
able.

In the transformation from trochophore to veliger, the
prototroch develops a powerful swimming-feeding organ, the
velum, which when expanded measures about 60-70 y.m in
diameter. Simultaneously the larva secretes the two D-shaped
calcareous valves of the shell (the straight-hinged pro-
dissoconch I stage, 60-70 ^ m in length) that completely clothe
and protect the soft organs of the retracted larva. While the
veliger is swimming it projects the velum between the valves,
but retracts it inside the valves at the slightest disturbance.
Two rows of long cilia, the preoral and pastoral, fringe the
margin of the velum, and propel the swimming veliger— a con-
tinuous activity during earlier veliger stages. A row of shorter
cilia, the adoral, lies between the preoral and postoral rows
and leads to the mouth on the posterior side of the velum
(Galtsoff, 1964; Elston, 1980; Waller, 1981).

FEEDING

The process of veligers feeding is understood only in
a rudimentary way (J orgensen, 1966; Easton, 1980; Kennedy
and Breisch, 1981; Waller, 1981). Particles are gathered by
preoral and postoral rows of long cilia, passed to the row of

CARRiKER: OYSTER LARVAE AND SUSPENDED PARTICLES

43

shorter adoral cilia between them, and those accepted as food
are carried posteriorly to the labial palps and the mouth. Re-
jected particles are dropped off at the site of the future foot
rudiment. The entire central subvelar region of the velum is
devoid of cilia. Strathmann and Leise (1979) hypothesized
that long cilia of the outer preoral band beat with a stiff straight
stroke and a curved return stroke to produce the major water
current used for both feeding and swimming. Particles are
carried in this current and when they come within reach of
the preoral cilia they are swept directly onto the cilia of the
adoral food groove and carried toward the mouth. Postoral
cilia beat towards the preoral cilia and the water current so
generated helps to capture food particles. Mucus picked up
by preoral cilia in the recovery stroke could aid in food cap-
ture. This opposed ciliary band system is thought to promote
effective food capture.

Nelson (1921) saw veligers swim to a mass of slime
scraped from a submerged log, and remaining stationary, with
the velum in contact with the mass, direct a strong current
of water past the mouth while gathering small food particles.
He also reported that during early larval life in relatively quiet
water veligers spend much time close to the surface, obtain-
ing food particles from finely divided detritus and microscopic
plants and animals in the surface film. Waller (1981)
presented an excellently illustrated (SEM) account of cilia-
tion of the velum of veligers of Ostrea edulis Linne that ap-
pears to correspond anatomically to that of the velum of
Crassostrea virginica (Galtsoff, 1964). Elston’s (1980) publica-
tion provides histological information on the velum of C.
virginica. Straight-hinged veligers of Crassostrea virginica in-
gest food particles ranging in size from 1 to 10/xm; later stage
veligers, up to 1 8 /xm; and eyed veligers up to 30 /xm (Mackie,
1969; Langdon, 1983; Webb and Chu, 1982). According to
Bayne (1976, 1983) veligers possess ciliary rejection currents
on the oral palps around the mouth, and rejection seems to
be based on the concentration and size of particles rather
than on any property or quality of the item as food. He ques-
tions whether bivalve veligers can distinguish between par-
ticles of similar size, shape, and density. Mackie (1969),
however, found the converse to be true for veligers of C.
virginica , and several authors have reported that post-set
oysters preferentially ingest algae and reject particulate in-
organic material (Loosanoff, 1962; Foster-Smith, 1978;
Newell, 1982). The subject clearly requires further research,
particularly on veligers in heavy concentrations of suspend-
ed materials. The selection process is probably complicated
by the presence in the mixture of suspended materials of
small inorganic particles coated with bacteria, small protozoa,
and other microorganisms (see for example, Odum, 1968).

In the presence of heavy loads of suspended particles,
oyster veligers could probably eject particles from the man-
tle cavity by rapid closure of the valves as is done by post-
set oysters. Though this behavior could account for some ex-
pulsion of particles, it is clearly not enough to protect veligers
in heavy concentrations of suspended sediments. Loosanoff
(1962) and Davis and Hidu (1969b), for example, showed the
growth of oyster veligers was significantly impaired at a con-
centration of 0.75 g // of silt, and larvae ceased to grow at

levels of 2 g // and higher. All veligers eventually died at con-
centrations of 3 gll and higher. Growth rate declined as the
size of suspended particles decreased, the largest particles
interferring least with growth. Silt maximally changed the pH
of seawater from 7.5 to 6.4, but survival and growth of lar-
vae at the low pH in clear seawater was better than that in
the presence of suspended silt (Calabrese and Davis, 1966).
Thus part of the deleterious effect of silt could be attributed
to the lowering of pH in the culture water, a matter needing
further research. On the other hand, veligers tolerated kaolin
and Fuller’s earth more than silt, showing appreciable growth
in a concentration of 2 gll of kaolin and fair survival but no
appreciable growth in 2 gll of Fuller’s earth.

In concentrations of 0.5 gll and lower, silt, kaolin, and
Fuller’s earth had a beneficial effect, often increasing the
growth rate of veligers over that of controls in clear seawater,
possibly because suspended particles chelated or adsorbed
toxins present in the larval cultures (Loosanofff, 1962; Davis
and Hidu, 1969b). However, in low concentrations (0.5 gll )
of silicon dioxide particles (sand, up to 5 /xm in size), veligers
suffered severe mortality; whereas in concentrations as great
as 4 gll of larger particles (5-25 and 25-50 /xm) of sand there
was not significant mortality (Davis and Hidu, 1969b). The
apparent explanation for this effect is that veligers filled their
stomachs with the fine sand particles, unable to reject them,
while larger particles were too large to pass down the
esophagus.

Unfortunately the behavior of veligers among sus-
pended particles under these various conditions was not
described, but the studies are significant in showing that the
degree of tolerance of oyster veligers to concentrations of
suspended materials is considerably in excess of normal am-
bient conditions in estuaries and coastal waters, and that low
concentrations of some turbidity producing materials can be
beneficial while others may be harmful. The type of native
habitat of oysters could make a difference in the tolerance
of their larvae to suspended materials; those living in highly
turbid waters, like those of the Carolinas, for example, would
be more tolerant than those coming from the relatively clear
waters of, for instance, Long Island Sound (Loosanoff, 1962).
This is an important consideration relative to the effects of
dredging and suggests the need for comparative geographic
investigations on the tolerance of veligers from both clean
and turbid waters to various kinds of suspended sediments.

As would be expected, the appropriate diameter of the
esophagus of straight-hinged and eyed veligers (illustrated
by Galtsoff, 1964, in Figures 336 and 340 respectively, and
allowing for distensibility of the functioning organ) readily ac-
commodates the size of food particles swallowed by these
larvae. Potentially suspendable particles in estuaries range
in size from that of colloids (5 to 5,000 A), to clays (less than
4 /xm), silts (4 to 62 /xm), sands (62 to 2,000 /xm) and larger
detrital particles and fecal pellets (3 mm or more) (Haven and
Morales-Alamo, 1968, 1970; Riley, 1970; Moore, 1977; Biggs,
1978). In lower Delaware River more than 97% of suspend-
ed sediment is within the silt and clay range (Biggs, 1978),
and in the James River Estuary 97% of suspended particles
are less than 44 /xm in size (Haven and Morales-Alamo, 1 968).

44

LARVAL ENTRAINMENT

A reasonable ratio for the size distribution of suspended
sediments in mid-Atlantic estuaries is 10% sand, 45% silt,
and 45% clay (Biggs, 1978).

Consequently, oyster veligers during their planktonic
ontogeny are surrounded by a wide range of sizes and kinds
of inorganic particles, the majority of which, clays and silts,
correspond in size roughly to that of the organic particles con-
sumed for food. Under highly turbid conditions when a greater
than normal concentration of inorganic particles is present
in the water, concentration of food particles to non-food par-
ticles could be extremely low. Even if veligers are able to sort
food particles from inorganic particles to some degree
(Mackie, 1969), under these extreme conditions the adoral
ciliary food tract, palps, mouth, and esophagus could become
clogged, partially or totally impairing food gathering and in-
gestion. Furthermore, ciliated epithelia of molluscs are well-
known for secreting copious quantities of mucus when ir-
ritated, and this mucus entangled with particles would fur-
ther add to congestion around the velum and mouth.
Response of the veliger, at whatever planktonic stage, would
probably be to close the valves and sink to the bottom where
probable burial in the sediment could occur. The critical
question posed by this hypothetical model concerns the max-
imal concentration of different sizes and kinds of suspended
inorganic and organic materials that different developmen-
tal stages of veligers can tolerate while continuing to feed
and grow. Studies reported by Loosanoff (1962) and Davis
and Hidu (1969b) provide a good beginning; much more
research, however, is required to answer this important ques-
tion fully.

Veligers possess a ciliated apical sense organ
unlerlain by the cerebral ganglion located in the central basal
part of the velar cup (Galtsoff, 1964; Elston, 1980; Waller,
1981). The function of this sense organ has not been dis-
covered, but should be studied in view of its possible role
in controlling selection and ingestion of food particles and
response of veligers to high levels of suspended materials.
Other possible anatomical structures for sensing suspend-
ed particles could be located on the mantle edge, gill
rudiments, and fringes of the velum. In this connnection, the
shell of prodissoconch II veligers possesses a conspicuous
fasciole and notch (Carriker and Palmer, 1979) that increases
in size as the larva grows, and terminates abruptly at the junc-
ture of the veliger and spat shell, i.e., during metamorphosis.
Waller (1981) found a similar structure in the larva of Ostrea
edulis which he named the posterodorsal notch. He associ-
ated the notch with a postanal tuft of cilia located just dorsal
to the anus that he suggested propels water out of the man-
tle cavity. Cilia of the postanal notch project between valves
adjacent to the notch.

Potential importance of the organ in the response of
veligers to suspended particles, as well as in setting activities,
should not be overlooked.

The feeding behavior of veligers in the presence of
high concentrations of inert particles in a range of tempera-
tures and salinities has not been explored. Veligers tend to
congregate in greatest concentration at the top of the
halocline; the sharper the salinity gradient between the over-

lying brackish water and the underlying more saline water,
the more marked the concentration of larvae when they are
present in the water mass (Nelson, 1931). However, response
of larvae to this salinity gradient in a range of concentrations
of suspended particles and at different temperatures should
be examined.

SHELL DEVELOPMENT

Biomineralization is an integral physiological function
of larval development in shelled molluscs. Under normal con-
centrations of suspended particles it appears that ultrascopic
particles in the water can be incorporated in the shell of post-
set (dissoconch) oysters (Carriker and Palmer, 1979). These
particles become a part of the developing shell edge as
seawater passes over the mantle lobes into the mantle cavi-
ty. In swimming veligers, however, most seawater passes
over the velum, and not necessarily into the mantle cavity
(a point that needs verification), so that presumably most par-
ticles in turbid waters simply slide around veliger valves. What
effects highly turbid waters could have on the normal, highly
complex development of the shell of veligers needs study.
Probably even gentle bombardment of the mantle edge by
clays and silts, not enough to cause the veliger to more than
partially withdraw its mantle edges, could slow shell deposi-
tion and growth (Moore, 1977). The larval shell affords pro-
tection to the larva, serves as a supportive external skeleton
within which the fleshy organs function, and transforms
through intricate metamorphic changes into the spat shell.
Consequently, abnormal shell growth could have serious con-
sequences on larval behavior and physiology, and ultimate-
ly metamorphosis to the spat stage.

LIGHT

The fact that early stages of veligers are somewhat
uniformly distributed vertically in the water column (Carriker,
1 951 ; Kunkle, 1 957) suggests that their distribution is not in-
fluenced by changes in day-night illumination (see also Thor-
son, 1964). However, within 24 hours of setting, mature
veligers develop a pair of dark pigment eyes sensitive to light
(Nelson, 1926), and these eyed larvae appear to be light sen-
sitive. Haskin (1964) reported that native mature and eyed
larvae from Delaware Bay react strongly to light in the
laboratory, responding to changing salinities only under light
passing through a yellow-grain filter with maximum transmis-
sion at 575 /im. Larvae also show a definite preference for
setting on the nonilluminated surfaces of control shells, and
setting intensity increases from a ratio of 4:96 to 20:80 when
illumination is reduced from 50 to 25 foot candles (Ritchie
and Menzel, 1969). The intensity of penetration of solar
energy in water is attenuated and the degree of penetration
of its spectral parts is modified by suspended particles. The
effect of high turbidities in the water column on the intensity
and spectral components of light that could affect the ver-
tical distribution of late stage veligers has not been examin-
ed. As reported by Ritchie and Menzel (1969), the shading
effect of a canopy of suspended particles alone could in-
fluence larva! behavior— if nothing more, increasing setting
intensity. Some turbidity could also serve as a screen to

CARRIKER: OYSTER LARVAE AND SUSPENDED PARTICLES

45

protect gametes and larvae from the injurious effects of
ultraviolet radiation near the surface (Wilber, 1971). This in-
teresting possibility has not been researched.

OXYGEN

Agitation of bottom sediments increases the rate of up-
take of oxygen by the organic component of sedimentary par-
tides by at least tenfold (Seattle University, 1970). The in-
itial oxygen uptake rate is maximal. Thus oxygen demand
of disturbed bottom sediments could significantly reduce the
dissolved oxygen concentration in estuarine waters during
periods of dredging. This is a particularly serious matter in
estuaries where tidal action keeps resuspended particles in
suspension for prolonged periods of time. Little information
is available on the oxygen requirements of oyster vefigers.
Black (1962) measured a ninefold increase in the rate of ox-
ygen consumption from first cleavage to the development of.
the trochophore. Since veligers become increasingly active
as they grow, it is likely that their need for oxygen increases
correspondingly (Crisp, 1975). It follows that in a highly tur-
bid aqueous environment veligers could not only be stressed
by effects of suspended particles but also by a potentially
serious oxygen deficit. However, evidence that oxygen de-
mand by the larvae exceeds what might be available, is
lacking.

PARTICLES

The physical impact of the range of sizes of particles
on oyster veligers has not been studied. Veligers of
Mercenaria mercenaha (linne), the hard clam, even when
crowded in laboratory cultures to densities of 50 larvae per
cubic centimeter of seawater seldom collide with each other
(Carriker, 1961), currents originated by cilia on the velum
seeming to act like the “slip stream of an aeroplane” (Turner
and George, 1955). Oyster larvae behave in much the same
way under these conditions. In denser cultures, collisions are
frequent, and some of the larvae fall toward the bottom.
Largest silt particles and smaller sand particles, when thrown
into the water column by sufficiently turbulent currents in the
wake of dredging or other mechanical disturbance of the bot-
tom in low concentrations would probably simply slide by lar-
vae. In dense concentrations of large particles, however, the
chance of collisions between particles and larvae would pro-
bably greatly increase, conceivably irritating veligers to close
and momentarily sink or fall to the bottom with the possibili-
ty of burial and suffocation.

VERTICAL MOVEMENTS

As they grow, larvae of Crassosirea virginica develop
stouter shells with conspicuous umbones (prodissoconch II
larvae, 82 to 284 ;tm long in New Jersey estuaries (Carriker,
1951). In New Jersey estuaries early umboned larvae (dur-
ing the first week of planktonic existence) remain more or less
uniformly distributed throughout the water column on both
flood and ebb. Late umboned veligers (mature and eyed
stages) tend to congregate on or near the bottom at both slack
periods and during at least parts of ebb flow, and to rise into
the water column during early and maximal flood (Nelson,

1931; Carriker, 1951, 1967; Kunkle, 1957; Haskin, 1964).
Alternative explanations for vertical movements of oyster lar-
vae in other parts of the world are given by Prytherch (1 928),
Korringa (1952), Manning and Whaley (1954), Verwey (1966),
Wood and Hargis (1971), and Andrews (1979), with some of
these explanations perhaps reflecting the influence of other
species of oysters (for example, Ostrea edulis ) and/or other
physiographic and hydrographic conditions. Near or on the
bottom, veiigers encounter suspended sediments con-
siderably more concentrated than in the upper strata of the
water column. How larvae react to these heavier concentra-
tions under normal turbidities is not known. In excessive con-
centrations of particles it is likely they close and settle to the
bottom. The matter should be investigated. Whether larvae
can remain closed on the bottom for long periods of time or
until the water clears sufficiently for them to swim back into
the water column, also remains unclear. How long larvae can
remain closed under adverse conditions is likewise unknown.
Particles immediately above the bottom during tidal flow move
by rolling, sliding, and jumping. Reaction of late- stage veligers
to this physical activity should also be investigated, and has
a bearing on survival of larvae during excessive rolling of bot-
tom sediments by mechanical or stormy conditions.

Bivalve larval swimming, controlled by beat of large
marginal velar cilia, is surprisingly variable. Periods of verti-
cle rising are interspersed with periods of sinking. Veligers
can sink by withdrawing the velum between the valves, by
actively swimming downwards or by sinking more slowly with
the velum uppermost and the velar cilia beating to retard the
rate of fall. Normal vertical swimming is performed with the
shell lowermost and velum uppermost (Carriker, 1961 ; Bayne,
1976). Non-directed swimming speeds range from less than
1 cm/min for early veligers to 5 cm/min for eyed larvae (Hidu
and Haskin, 1978). When subjected to hourly salinity in-
creases of 0.5% in the laboratory, most larvae swim upward
or downward at approximately three times these speeds.
Veligers with valves closed sink at speeds of 5 to 50 cm/min
depending on the stage of larval development. The predomi-
nant swimming behavior is a slow spiral at a maximal rate
of 5 cm/min at 25°C associated with maintenance of posi-
tion. Less common are upward and downward movements
at speeds up to 14 cm/min.

HORIZONTAL MOVEMENTS

Investigations in New Jersey estuaries suggest that
mature and eyed veligers of Crassostrea virginica control their
horizontal distribution by regulating their vertical position
relative to direction of tidal currents, moving up the estuary
by successive stages in the non-tidal bottom up-estuary flow,
in some cases far beyond the location of the parents (Nelson,
1921, 1931, 1955; Carriker, 1951, 1967; Haskin, 1964; Ken-
nedy and Breisch, 1981). While straight-hinged and early um-
boned veligers remain more or less uniformly distributed ver-
tically in the water column (Kunkle, 1957), older veligers drop
into lower strata of the water column. In some cases large
concentrations of mature and eyed larvae have been found
on or near the bottom during ebb tide (Carriker, 1951). In
laboratory experiments Haskin (1964) showed that mature

46

LARVAL ENTRAINMENT

and eyed larvae collected in Delaware Bay are responsive
to changing salinities, and suggested that salinity variations
can be playing a dominant role in larval activity in nature even
in the absence of a halocline or salt-wedge. Available infor-
mation indicates that response of planktonic larvae of
estuarine benthic invertebrates to non-tidal drift circulation
of estuarines could be widespread and even extend to en-
tirely pelagic estuarine species (Carriker, 1967). But not all
investigators are in accord with this theory (the “Nelson
School”) (Korringa, 1952; Andrews, 1979, for example), pro-
bably because they have dealt with other species of oysters
or in bodies of water of different hydrographic conditions (for
example, Prytherch, 1928). There is, nonetheless, quite pro-
bably a behavioral mechanism that takes older veligers up
the estuary in non-tidal drifts; what this mechanism is, and
how it is affected by heavy loads of suspended particles have
yet to be determined. If up-estuary, non-tidal movements of
late-stage veligers are stimulated primarily by salinity changes
(Haskin, 1964), albeit minor ones under some hydrographic
conditions, it is likely that dense concentrations of particles
near and over the bottom could seriously interfere with swim-
ming and other physiological functions of veligers, in part or
totally eclipsing the stimulatory changes in salinity and cur-
rent flow and the capacity of the veligers to rise into the water
column on the flooding tide. Horizontal movements of bivalve
larvae, especially relative to possible interference from
suspended particles, offer several challenging research
problems.

POISONS

Suspended particles can poison veligers through tox-
ic compounds adsorbed onto the particles (Kennedy and
Breisch, 1981). Accelerated industrial activity has lead to in-
creased dumping of a wide range of kinds of poisonous com-
pounds into estuaries and coastal waters (Cunningham and
Tripps, 1973). These compounds can remain in solution or
become adsorbed and concentrated especially on finely par-
ticulate clay minerals (Kerr and Vass, 1973; Fuller, 1974),
some of which subsequently could become incorporated in
organic aggregates (detritus, Riley, 1970) and sink to the bot-
tom. Stirred into the water column by natural or human ac-
tivities, these particles, as well as finer ones remaining in
suspension, come in contact with epithelia of swimming
veligers in the water mass. If ingested by larvae, contaminated
particles can be stripped of adsorbed poisons in the larval
gut; if in solution, poisons can come in direct contact with
veliger epithelia where they can be adsorbed; or suspended
particles can touch the epithelia passing poisons by contact.
In any, or all of these cases, poisons injure or kill the larvae
when present in lethal concentrations. Sublethal doses un-
doubtedly disrupt normal behavior, create aberrant swimming
movements and feeding, and result in loss of vigor, poor
growth, and increased susceptibility to disease and preda-
tion. Whether larvae can reject poisoned particles, as they
might non-food particles, is an interesting question, but cer-
tainly they cannot avoid toxic compounds in solution in the
water immediately around them.

Earliest life history stages of oysters (eggs, embryos)

tend to be the least resistant to toxic substances. Mercury,
silver, copper, and zinc are especially toxic to embryos
(Calabrese et ai, 1 973). Also highly toxic are crude oils (Ren-
zoni, 1975), pesticides (Davis and Hidu, 1969a), petroleum
hydrocarbons, carcinogens and mutagenic chemicals (Neff,

1979) , and chlorine produced oxidants (Roosenburg et at.,

1980) . Embryos and larvae are no more tolerant to biode-
gradable (linear alkylate sulfonates) than to older
nondegradable detergents; a concentration of 0.1 mg//
resulted in a mortality of 36% of larvae tested, and many sur-
vivors were abnormal in shape and size (Calabrese, 1972).
In another example, 50% of straight-hinged veligers were
killed in a 48 hour exposure to a concentration of only 0.3
ppm of chlorine-produced oxidants (Roosenburg et ai, 1 980).
Beyond the individual effects of poisons, there are unknown
but probably serious synergistic effects resulting from the pot-
pourri of poisons currently identified in our estuaries and
coastal waters— effects causing increasing concern among
marine biologists, fisheries scientists, and others.

PEDIVELIGERS

INITIAL ATTACHMENT

The eyed pediveliger (Carriker, 1961) retains the strong
swimming and food-collecting velum of the veliger and
develops a highly functional foot (about 300-400 /xm in length
when extended) on which it crawls smoothly over the
substratum in search of a setting site. During the search the
pediveliger alternates velar swimming and pedal crawling,
free to depart from surfaces that are unsuitable for setting
(Bayne, 1976). While swimming it extends its foot between
the valves in the direction of swimming, the tip of the foot
frequently turning right or left and up or down, apparently
serving to orient larval swimming movements. When the foot
touches a solid surface, the pediveliger stops swimming and
partially withdraws the velum; to leave the surface the larva
extends the velum and swims away.

Pediveligers appear to settle more commonly in shad-
ed areas, as on the underside of shells and other surfaces,
than in lighted areas (Ritchie and Menzel, 1969), supporting
Nelson’s (1 926) observations that the eye spots are sensitive
to light (Haskin, 1964). Other studies of settlement in rela-
tion to light have been contradictory for reasons that are not
clear (Kennedy and Breisch, 1981). Pediveligers respond to
proteinaceous compounds on the surface of oyster shells
(Crisp, 1967), and setting appears to be initiated through the
action of a waterborne pheromone (Hidu, 1969; Veitch and
Hido, 1971; Hidu et ai, 1978). They are attracted to settle
in small pits and other irregularities in shell and other hard
surfaces.

While searching for a permanent setting site on a clean
substratum (the surface of an oyster shell, for example), the
pediveliger extends the foot, touches the tip temporarily to
the surface, and pulls the body over the contracting foot. The
direction of crawling changes and occasionally reverses.
Creeping continues for some time, the area of search gradual-
ly becoming smaller and the crawling slower. When the
specific spot is located, the pediveliger extends the foot far

CARRIKER: OYSTER LARVAE AND SUSPENDED PARTICLES

47

beyond the shell, secretes a clear, fine byssus and fixes it
to the substratum, then turns itself on its left side, the valve
touching the substratum. This action is followed by discharge
from the byssal gland of a tanned protein cement that is
passed between the left valve and the substratum. This
adhesive sets in the water in a few minutes and permanent-
ly attaches the pediveligerto the substratum (Prytherch, 1934;
Galtsoff, 1964; Cranfield, 1973; Tomaszewski, 1981).

Impact of suspended sediments on pediveliger settle-
ment and metamorphic processes is unclear, except that a
deposit of loose sediment only 1 to 2 mm thick is thought to
be enough to render hard surfaces unsuitable for attachment
(Galtsoff, 1964). Considering that setting size of pediveligers
is only about a third of a millimeter, it is probable that layers
of sediment even considerably less than 1-2 mm can seriously
impede settlement.

The fact that pediveligers can attach to surfaces
covered with thin layers of mucoid materials, microorganisms,
and detritus raises the question of whether they can use the
foot in any way and to any extent clean the surface of loose
particles prior to cementing the left valve to it, the byssal
thread serving as an anchor line during the processes. This
possible behavior has not been investigated. Should a
pediveliger attempt attachment to a layer of loose sediment
on a hard surface, its cement probably would not adhere to
the substratrum, leaving the bivalve vulnerable to flushing
from the surface. Furthermore, since the foot possesses
rather little (cilia and mucus) on its surface to provide trac-
tion while exploring the setting side, it is likely the pediveliger
can set only in relatively low to moderate current velocities.
The foot possesses a musculature (in addition to cilia and
the possible adhesive qualities of the mucus) that could aid
in traction, as in so many gastropods, but this matter remains
unexplored.

The highly adsorptive qualities of mineral particles
could alter, and even block, the attraction to larvae of pro-
teinaceous substances on the surface of bivalve shells, as
well as to waterborne phenomones (Crisp, 1967; Hidu, 1969).
Pits and other irregularities on hard surfaces, coated by a
thin layer of sediment, likewise would be unrecognizable by
the larvae.

POST ATTACHMENT

After settlement the eyed pediveliger undergoes loss
of the velum through fragmentation and swallowing of some
of the pieces; resorption of the eyed spots, foot, and anterior
adductor muscle by phagocytic activity; development of the
palps and gills, and a reorientation of the organs in the man-
tle cavity (Galtsoff, 1964; Bayne, 1976). Degeneration of the
velum and formation of the palps occurs within one to three
days in mussel larvae (Bayne, 1976) and probably in the same
period in oyster larvae. During the metamorphic period, while
the velum is fragmenting and the palps and gills are develop-
ing, the larva cannot feed and relies for metabolic energy on
stored nutrients (Bayne, 1976; Manahan and Crisp, 1982).

Immediately after pediveligers settle, thin layers of
sediment deep enough only to partially cover them could in-
terfere with respiration (especially during the change from

velum to palps and gills) and with the shift in secretion by
the mantle of the shell from larval aragonite to spat calcite
(Carriker and Palmer, 1979). Irritation of ciliated epithelia on
the developing gills and palps and mantle surfaces by
suspended particles would characteristically stimulate secre-
tion of copious quantities of mucus to cleanse these surfaces
possibly resulting in something of a physiological drain and
interfering with respiration. Under these conditions rate of sur-
vival of spat would depend, no doubt, on thickness of the layer
of sediment and its persistence, nature of particles (whether
predominantly clays, silts, fine sands, or organic aggregates,
or various combinations of these), extent of clumping, amount
of mucoid binding substances, presence of toxic substances
adsorbed to particles (as well as in solution) (McGeer, 1982),
and duration of contact of these substances with the free
ciliated surfaces of metamorphosis juveniles.

RECAPITULATION

Since oysters and their larvae have adapted over a
long geologic period to highly successful survival in an
ecosystem characterized by dominance of natural, suspend-
ed particles, it follows that they can tolerate, and in low con-
centrations benefit, from these materials. The present syn-
thesis clearly demonstrates, however, that a high concentra-
tion of many kinds of suspended particles can be harmful to
embryos, veligers, and pediveligers of Crassostrea virginica.
The dividing range of concentrations of particles between
beneficial and harmful, however, has not been determined
with any precision. What can be reported from helpful, but
preliminary investigations, is that beneficial concentrations
for embryos in the egg membrane are below 0.25 g //, and
for veligers below 0.75 g //. The optimal range of concentra-
tions of particles for normal physiological activity and behavior
for the different stages in the larval ontogeny has yet to be
determined.

Current success of bivalve larval culture in the
laboratory (Bolton, 1982; Pruderefa/., 1982) and techniques
available for study of the physiology and behavior of
microscopic animals under controlled laboratory conditions
auger well for success in the determinations of beneficial and
harmful ranges of concentrations of a wide variety of natural
and harmful particles for the different ontogenetic stages of
bivalve larvae. Although addition of limited concentrations of
particulate inorganic matter improves growth in bivalves,
little is understood of the mechanism(s) whereby this occurs
(Loosanoff, 1962; Davis and Hidu, 1969b; Pruderefa/., 1982;
AN, 1982). These observations open an important field for fur-
ther research in mariculture.

LITERATURE CITED

Ali, S. M. 1982. Effect of natural silt on oyster growth. In: Proceedings
of the Second International Conference on Aquaculture Nutri-
tion: Biochemical and Physiological Approaches to Shellfish
Nutrition. G. D. Pruder, C. J. Langdon and D. E. Conklin, eds.,
pp. 431-432. Louisiana State University, Baton Rouge.
Andrews, J. D. 1979. Pelecypoda: Ostreidae. In: Reproduction of

48

LARVAL ENTRAINMENT

Marine Invertebrates, Vol. 5, Molluscs: Pelecypods and Lesser
Classes. A. C. Giese and J. S. Pearse, eds. pp. 293-341.
Academic Press, New York.

Bayne, B. L., ed. 1976. Marine Mussels: Their Ecology and Physiology.
Cambridge University Press, London, 506 p.

Bayne, B. L. 1983. Physiological ecology of marine molluscan lar-
vae. In: The Mollusca, Development, N. H. Verdonk, J. A. M.
van den Biggelaar, and A. S. Tompa, eds. pp. 299-343. Vol.
3, Academic Press, New York.

Biggs, R. B. 1978. Coastal bays. In: Coastal Sedimentary En-
vironments, R. A. Davis, Jr., ed. pp. 67-99. Springer-Verlag,
New York.

Black, R. E. 1962, Respiration, electron-transport enzymes, and
Krebs-cycle enzymes in early development stages of the oyster
Crassostrea virginica. Biological Bulletin 123:58-70.

Bolton, E. T. 1982. Intensive Marine Bivalve Cultivation in a Controlled
Recirculating Seawater Prototype System. University of
Delaware Sea Grant College Program, Newark, Delaware.
DEL-SG-07-82, 165 pp.

Calabrese, A. 1972. How some pollutants affect embryos and lar-
vae of the American oyster and hard-shell clam. Marine
Fisheries Review 34:66-77.

Calabrese, A. and H. Davis. 1966. The pH tolerance of embryos and
larvae of Mercenaria mercenaria and Crassostrea virginica.
Biological Bulletin 131:427-436.

Calabrese, A., R. S. Collier, D. A. Nelson and J. R. Macinnes. 1973.
The toxicity of heavy metals to embryos of the American
oyster, Crassostrea virginica. Marine Biology 18:162-166.

Carriker, M. R. 1951. Ecological observations on the distribution of
oyster larvae in New Jersey estuaries. Ecological Monographs
21:19-38.

Carriker, M. R. 1961. Interrelation of functional morphology, behavior,
and autecology in early stages of the bivalve Mercenaria
mercenaria. The Journal of the Elisha Mitchell Scientific Society
77:168-241.

Carriker, M. R. 1967. Ecology of estuarine benthic invertebrates: a
perspective, in: Estuaries, G. H. Lauff, ed. pp. 442-487.
American Association for the Advancement of Science,
Washington, DC, Publication No. 83.

Carriker, M. R. and R. E. Palmer. 1979. Ultrastructural mor-
phogenesis of prodissoconch and early dissoconch valves of
the oyster Crassostrea virginica. Proceedings of the National
Shellfisheries Association 69:102-128.

Cranfield, H. J. 1973. Observations on the behavior of the pediveliger
of Ostrea edulis during attachment and cementing. Marine
Biology 22:203-209.

Crisp. D. J. 1967. Chemical factors inducing settlement in Crassostrea
virginica (Gmelin). Journal of Animal Ecology 36:329-335.

Crisp, D. J. 1975. The role of the pelagic larva. In: Perspectives in
Experimental Biology, P. Spencer Davies, ed. pp. 145-155. Vol.
1, Pergamon Press, Oxford.

Cunningham, P. A. and M. R. Tripp. 1973. Accumulation and depura-
tion of mercury in the American oyster Crassostrea virginica.
Marine Biology 20:14-19.

Davis, H. C. and H. Hidu. 1969a. Effects of pesticides on embryonic
development of clams and oysters and on survival and growth
of the larvae. Fishery Bulletin, U. S. Fish & Wildlife Service
67:393-403.

Davis, H. C. and H. Hidu. 1969b. Effects of turbidity-producing
substances in sea water on eggs and larvae of three genera
of bivalve mollusks. Veiiger 11:316-323.

Elston, R. 1980. Functional anatomy, histology and ultrastructure
of the soft tissue of the larval American oyster, Crassostrea
virginica. Proceedings of the National Shellfisheries Associa-

tion 70:65-93.

Foster-Smith, R. L. 1978. The function of the pallia! organs of bivalves
in controlling ingestion. Journal of Molluscan Studies 44:83-99.

Fuller, S. L. H. 1974. Clams and mussels (Mollusca: Bivalvia). In:
Pollution Ecology of Freshwater Invertebrates, C. W. Hart, Jr.
and S. L. H. Fuller, eds. pp. 215-273. Academic Press, New
York.

Galtsoff, P. S. 1964. The American oyster Crassostrea virginica
Gmelin. Fishery Bulletin 64:1-480.

Haskin, H. H. 1964. The distribution of oyster larvae. Proceedings
Symposium on Experimental Marine Ecology, Occasional
Publication No. 2, Graduate School of Oceanography, Univer-
sity of Rhode Island, pp. 76-80.

Haven, D. S. and R. Morales-Alamo. 1968. Occurrence and transport
of faecal pellets in suspension in a tidal estuary. Sedimentary
Geology 2:141-151 .

Haven, D. S. and R. Morales-Alamo. 1970. Filtration of particles from
suspension by the American oyster Crassostrea virginica.
Biological Bulletin 139:248-264.

Hidu, H. 1969. Gregarious setting in the American oyster Crassostrea
virginica Gmeliln. Chesapeake Science 10:85-92.

Hidu, H. and H. H. Haskin. 1978. Swimming speeds of oyster lar-
vae, Crassostrea virginca, in different salinities and
temperatures. Estuaries 1:252-255.

Hidu, H., W. G. Valleau and F. P. Veitch. 1978. Gregarious setting
in European and American oysters— response to surface
chemistry versus water-borne phenomena. Proceedings of the
National Shellfisheries Association 68:11-16.

Jorgensen, C. B. 1966. Biology of Suspension Feeding. Pergamon
Press, New York, 357 pp.

Kennedy, V. S. and L. L. Breisch. 1981 . Maryland’s Oysters: Research
and Management. University of Maryland Sea Grant Publica-
tion No. UM-SG-T3-81-04, 286 pp.

Kerr, S. R. and W. P. Vass. 1973. Pesticide residues in aquatic in-
vertebrates. In: Environmental Pollution by Pesticides, C. A.
Edwards, ed. pp. 134-180. Plenum Press, New York.

Korringa, P. 1952. Recent advances in oyster biology. Quarterly
Review of Biology 27:274-308, 339-365.

Kunkle, D. R. 1957. The vertical distribution of oyster larvae in
Delaware Bay. Abstract, Proceedings of the National
Shellfisheries Association 48:90-91.

Langdon, C. J. 1982. New techniques and their application to studies
of bivalve nutrition. In: Proceedings of the Second International
Conference on Aquaculture Nutrition: Biochemical and
Physiological Approaches to Shellfish Nutrition, G. D. Pruder,
C. J. Langdon and D. E. Conklin, eds. pp. 305-320. Louisiana
State University, Baton Rouge.

Loosanoff, V. 1962. Effects of turbidity on some larval and adult
bivalves. Proceedings of the Gulf and Caribbean Fisheries In-
stitute, 14th Annual Session, November 1961, pp. 80-94.

Mackie, G. 1969. Quantitative studies of feeding in the oyster,
Crassostrea virginica. Abstract, Proceedings of the National
Shellfisheries Association 59:6-7.

Manahan, D. T. and D. J. Crisp. 1982. The role of dissolved organic
material in the nutrition of pelagic larvae: amino acid uptake
by bivalve veligers. American Zoologist 22:635-646.

Manning, J. H. and H. H. Whaley. 1954. Distribution of oyster lar-
vae and spat in relation to some environmental factors in a
tidal estuary. Proceedings of the National Shellfisheries
Association 48:56-65.

McGreer, E. R. 1982. Factors affecting the distribution of the bivalve
Macoma balthica on a mud flat receiving seawater effluent,
Fraser River Estuary, British Columbia, Canada. Marine En-
vironmental Research 7:131-150.

CARRIKER: OYSTER LARVAE AND SUSPENDED PARTICLES

49

Moore, P. G. 1977. inorganic particulate suspensions in the sea and
their effects on marine animals. Oceanography and Marine
Biology Annual Review 15:225-363.

Neff, J. M. 1979. Polycyclic Aromatic Hydrocarbons in the Aquatic
Environment. Sources, Fates, and Biological Effects. Applied
Science Publishers, London, 262 pp.

Nelson, B. W. 1962. Important aspects of estuarine sediment
chemistry for benthic ecology. In: Symposium on the En-
vironmental Chemistry of Marine Sediments. N. Marshall, ed.
pp. 27-41 . Occasional Publications No. 1 , Narragansett Marine
Laboratory, University of Rhode Island, Kingston.

Nelson, T. C. 1921. Aids to successful oyster culture. I. Procuring
the seed. New Jersey Agricultural Experiment Station Bulletin
351, 59 pp.

Nelson, T. C. 1 926. Report of the Department of Biology, Agricultural
Experiment Station, New Brunswick, New Jersey for the year
ending June 30, 1925, pp. 281-288.

Nelson, T. C. 1931. Annual report of the Department of Biology,
July 1, 1929- June 30, 1930. New Jersey Agricultural Experi-
ment Station, 47 pp.

Nelson, T. C. and J. B. Allison. 1940. On the nature and action of
diantlin, a new hormone-like substance carried by the sper-
matozoa of the oyster. Journal of Experimental Zoology
85:299-338.

Nelson, T. C. 1955. Observations on the behavior and distribution
of oyster larvae. Proceedings of the National Shellfisheries
Association 45:23-28.

Newell, R. I. E. 1982. Molluscan bioenergetics - a synopsis. In: Pro-
ceedings of the Second International Conference on
Aquaculture Nutrition: Biochemical and Physiological Ap-
proaches to Shellfish Nutrition, G. D. Pruder, C. J. Langdon,
and D. E. Conklin, eds. pp. 252-271. October 27-29, 1981,
Lewes-Rehoboth Beach, Delaware. Published by Louisiana
State University, Division of Continuing Education, Baton
Rouge.

Odum, W. E. 1 968. The ecological significance of fine particle selec-
tion by the striped mullet Mugil cephalus. Limnology and
Oceanography 13:92-98.

Pruder, G. D., C. J. Langdon and D. E. Conklin, eds. 1982. Pro-
ceedings of the Second International Conference on
Aquaculture Nutrition: Biochemical and Physiological Ap-
proaches to Shellfish Nutrition, October 27-29, 1981, Lewes-
Rehoboth Beach, Delaware. Published by Louisiana State
University, Division of Continuing Education, Baton Rouge,
444 pp.

Prytherch, H. F. 1928. Investigation of the physical conditions con-
trolling spawning oysters and the occurrence, distribution of
setting oyster larvae in Milford Harbor, Connecticut. Bulletin

of the United States Bureau of Fisheries Commission
44:429-503.

Prytherch, H. F. 1934. The role of copper in the setting, metamor-
phosis, and distribution of the American oyster, Crassostrea
virginica. Ecological Monographs 4:45-107.

Renzoni, A. 1975. Toxicity of three oils to bivalve gametes and lar-
vae. Marine Pollution Bulletin 6:125-128.

Riley, G. A. 1970. Particulate and organic matter in sea water. Ad-
vances in Marine Biology 8:1-118.

Ritchie, T. P. and R. W. Menzel. 1969. Influence of light on larval
settlement of American oysters. Proceedings of the National
Shellfisheries Association 59:116-120.

Roosenburg, W. H., J. C. Rhoderick, R. M. Block, V. S. Kennedy,
S. R. Gullans, S. M. Vreenegoor, A. Rosenkranz and C. Col-

lette. 1 980. Effects of chlorine-produced oxidants on survival
of larvae of the oyster Crassostrea virginica. Marine Ecology
Progress Series 3:93-96.

Sastry, A. N. 1979. Pelecypoda (Excluding Ostreidae). In: Reproduc-
tion of Marine Invertebrates. Vol. 5, Molluscs: Pelecypods and
Lesser Classes. A. C. Giese and J. S. Pearse, eds. pp.
113-292. Academic Press.

Seattle University. 1970. The oxygen uptake demand of resuspended
bottom sediments. Water Quality Office, Environmental Pro-
tection Agency. Superintendent of Documents, U. S. Govern-
ment Printing Office, 38 pp.

Stenzel, H. B. 1971 . Oysters. In: Treatise on Invertebrate Paleontology,
Part N, Vol. 3 (of 3), Mollusca 6, Bivalvla. R. A. Moore, ed.
pp. N953-N1224. Geological Society of America and Univer-
sity of Kansas.

Stephens, G. C. 1983. Dissolved organic material and the nutrition
of marine bivalves. In: Proceedings of the Second International
Conference on Aquaculture Nutrition: Biochemical and
Physiological Approaches to Shellfish Nutrition, October 27-29,
1981, Lewes-Rehoboth Beach, Delaware, G. D. Pruder, C. J.
Langdon, and D. E. Conklin, eds. pp. 338-357. Published by
Louisiana State University, Division of Continuing Education,
Baton Rouge.

Strathmann, R. R. and E. Leise. 1979. On feeding mechanisms and
clearance rates of molluscan veligers. Biological Bulletin
157:524-535.

Thorson, G. 1964. Light as an ecological factor in the dispersal and
settlement of larvae of marine bottom invertebrates. Ophelia
1:167-208.

Tomaszewski, C. 1981. Cementation in the early dissoconch stage
of Crassostrea virginica (Gmelin). Master’s Thesis, Universi-
ty of Delaware, 94 pp.

Turner, H. J. and C. J. George. 1955. Some aspects of the behavior
of the quahaug, Venus mercenaria, during the early stages.
Eighth Report on Investigation of the Shellfisheries of
Massachusetts, Department of Natural Resources, Division
of Marine Fisheries, Commonwealth of Massachusetts, pp.
5-14.

Veitch, F. P. and H. Hidu. 1971 . Gregarious setting in the American
oyster Crassostrea virginica Gmelin. 1. Properties of a par-
tially purified “setting factor.” Chesapeake Science
12:173-178.

Verwey, J. 1966. The role of some external factors in the vertical
migration of marine animals. Netherlands Journal of Sea
Research 3:245-266.

Waller, T. R. 1 981 . Functional morphology and development of veliger
larvae of the European oyster, Ostrea edulis Linne. Smithson-
ian Contributions to Zoology, No. 328, 70 pp.

Webb, K. L. and F. E. Chu. 1982. Phytoplankton as a food source
for bivalve larvae. In: Proceedings of the Second International
Conference on Aquaculture Nutrition: Biochemical and
Physiological Approaches to Shellfish Nutrition, October 27-29,
1981 , Lewes-Rehoboth Beach, Delaware. G. D. Pruder, C. J.
Langdon and D. E. Conklin, eds. pp. 272-291. Published by
Louisiana State University, Division of Continuing Education,
Baton Rouge.

Wilber, C. G. 1971. Turbidity, Animals. In: Marine Ecology, Vol. 1,
O. Kinne, ed. pp. 1181-1189. John Wiley & Sons, London.

Wood, L. and W. J. Harris, Jr. 1971. Transport of bivalve larvae in
a tidal estuary. In: Fourth European Marine Biology Symposium.
D. J. Crisp, ed. pp. 29-44. Cambridge University Press.

ARCTICA ISLANDICA (LINNE) LARVAE: ACTIVE
DEPTH REGULATORS OR PASSIVE PARTICLES1

ROGER MANN

VIRGINIA INSTITUTE OF MARINE SCIENCE
SCHOOL OF MARINE SCIENCE
COLLEGE OF WILLIAM AND MARY
GLOUCESTER POINT, VIRGINIA 23002, U.S.A.

ABSTRACT

The seasonal change in depth distribution of Arctica islandica (Linne) larvae at a station on
the Southern New England Shelf for the period April-December 1981 is compared with the output
of a numerical model designed to predict distribution in a scenario where active depth regulation
predominates. Larvae in excess of 200 fim length were present in the field in May at 1-30 m depth
and, at depths of 20-40 m from late July through November. The majority of larvae captured in November
were shelled veligers of 1 10 /xm length. Good agreement of the model with field data exists with respect
to absence of A. islandica larvae in the warm (> 18°C) shallow (0-20 m) waters between July and
early September, and the abundance of larvae throughout the depth range 20-40 m from July through
October. The model predicts occurrence of larvae in June; however, they were not seen in the field.
The discrepancy can be due to the combination of reduced spawning by adult A. islandica (which
is not included in the model) and less than optimum conditions for larval development. The model
predicts aggregation of the negatively geotactic larvae at the surface following decay of a seasonal
thermocline. Such aggregations were not seen in the field indicating that vertical mixing of the water
column in the fall months is sufficient to negate distribution patterns dominated by active depth regula-
tion. Depending upon the nature, intensity and stability of stratification of the water column, it is evi-
dent that depth distribution of A. islandica larvae can be dominated by either active depth regulation
or passive movement at the mercy of physical mixing. The conditions of transition from active to passively
dominated dispersal and distribution are poorly defined.

The literature relating to swimming behavior and
dispersal of bivalve larvae has been reviewed by Mann (1986).
Opinions differ as to whether dispersal is predominantly a
process of passive movement at the mercy of water currents
or a combination of active depth regulation coordinated with
horizontal stratification and flow. Resolution of the debate as
to the relative roles of active versus passive processes is con-
founded by the nature of the data sets available. An historical
emphasis on species of commercial importance dictated the
focus of major efforts on field studies in shallow estuarine
systems. Field studies in isolation are essentially observa-
tional and the type of the data collected allows only the in-
ference of cause and effect from correlation. Definitive resolu-
tion of cause and effect can only be effected by controlled
experiments. Furthermore, the physics of estuarine circula-
tion are so complex and dynamic that they too remain only
poorly understood. It is apparent then that ecologists have
chosen to examine the problem of bivalve larval dispersal in

Contribution # 1308 from the Virginia Institute of Marine Science.

perhaps the most intractable of environments and with only
a limited arsenal of approaches.

Recent work examining larval behavior in controlled
laboratory systems has, when combined with modelling, pro-
vided a new and powerful tool with which to address the pro-
blem of larval dispersal. This approach has been used in
studies of blue crab Callinectes sapidus (Rathbun) by Sulkin
and Van Fleuklem (1982) and red crab Geryon quinquedens
Smith by Kelly etal., (1982), but has not previously been ade-
quately exploited for bivalve molluscs. In this study I com-
pared the results of field observations of depth distribution
of the larvae of Arctica islandica (Linne) with a numerical
model, constructed solely from laboratory generated
behavioral data and field collected physical data. The model
was designed to predict A. islandica larval distribution in a
scenario where active depth regulation predominates. In ef-
fect the comparison allows discussion of the relative roles
of active and passive processes in dispersal. Field study data
are extracted from a larger data set collected in 1 981 during
a limited survey of seasonal occurrence, species composi-

American Malacological Bulletin, Special Edition No. 3(1986):51-57

51

52

LARVAL ENTRAINMENT

tion and depth distribution of bivalve larvae at a station on
the Southern New England Shelf (Mann, in press). While this
survey was of insufficient scale to examine both spatial and
inter-annual variability, it nonetheless permitted examination
of larval distribution in the relatively (compared to an estuary)
uncomplicated physical environment of the Southern New
England Shelf.

MATERIALS AND METHODS

A complete description of the study from which these
field data are extracted is given by Mann (in press). Only rele-
vant details are repeated here.

During the period April 13-December 14, 1981 (Julian
days 113-348), 14 one-day cruises were made to a 43 m deep
station WSW of Cuttyhunk Island, MA; west of Gay Head,
Martha’s Vineyard, MA; and east of Block Island, Rl (lat.
72°02’W, long. 41°14’N). The water column at this station ex-
hibits an intense seasonal stratification in temperature that
is representative of Southern New England Shelf and Mid-
dle Atlantic Bight waters (Mann, unpublished data). Adult Arc-
tica islandica are abundant in this area (Merrill and Ropes,
1969; Ropes, 1978; Fogarty, 1981). Depth specific plankton
tows were made, always during the hours of 1030-1430, at
1 m, 10 m, 20 m, 30 m, and 40 m with a Clarke-Bumpus net
(30 cm diameter, 5:1 aspect ratio, 53 /xm mesh, 10 minute
tow duration, 2 knots speed). Tows were not replicated.
Volume of water passing through the net was recorded
using a vane rotor in the mouth of the net. Volume sampled
varied between 9.64 and 10.28 m3 with a mean of 9.96 m3.
The collected sample was stained with Rose Bengal and fixed
with 10% v/v buffered formalin in sea water. Bivalve larvae
were subsequently separated under a low power dissecting
microscope. During periods of peak abundance plankton
samples were split using the apparatus of Drinnan and
Stallworthy (1979). Individual larvae were measured in length
(anterior-posterior axis) and height (dorso-ventral axis) at 100
x or 400 x on a Leitz compound microscope fitted with an
ocular micrometer. Larvae of 200 jxm length were identified
to species where possible. Larvae of A. islanica were iden-
tified using the key of Lutz et at., (1982). On each sampling
date temperature and conductivity were recorded at 5 m
depth intervals. Time-depth contour diagrams of each were
constructed by linear interpolation.

The four sets of data used for model development were
obtained as follows. A daily water temperature matrix at
5-meter depth intervals from surface to 45 meters was
generated from the previously described contour diagram.
Temperature specific growth rates of larval Arctica islandica,
expressed as daily increase in length at 9°G and 13-15°C,
were derived by linear interpolation of points given in Lutz
etal., (1982, Fig. 2). These points were fitted to passthrough
an origin corresponding to 90 /xm length (equal to the egg
diameter) at day 0 and mean length at metamorphosis of 260
/xm. Growth rate at 9°C was 3.09 /xm/day. Growth rate was
considered to be constant in the range 13-15°C at 5.31
/xm/day. Recent attempts to culture A. islandica larvae under
conditions identical to cultures 2-5 of Lutz et at., (1982) but

6 “1
S -

tr

o

x o 4 —
t— v.

S »

O o

(T x>

CD \

CE

< —
_1

0 -

TEMPERATURE °C

Fig. 1. Temperature specific growth rate model input data for Artica
islandica. The curve is developed from linear interpolation of growth
rate data taken at 6°C (Mann, unpublished), 9°C and 13-15°C (Lutz
et al., 1982) and 20°C (Landers, 1976). Curve type A and B differ
in point of inflexion between 15 and 20°C (see text).

at a lower temperature of 6°C, indicated that growth was ar-
rested at the onset of the shelled veliger stage and, despite
active feeding of the larvae, no larvae in excess of 120 /xm
length were recorded by 17 days after fertilization, when the
culture was terminated (Mann, unpublished data). Therefore,
growth for the present mode! was assumed to cease at 6°C.
The true shape of the growth curve between the aforemen-
tioned points is not known. Previous laboratory studies with
bivalve larvae (Walne, 1965; Helm and Millican; 1977) in-
dicated gradually increasing growth rate with temperature to
a maximum followed by a rapidly decreasing growth rate
above this optimum temperature. For the present study,
temperature specific growth rate was assumed to increase
linearly at 1.03 /xm/day/°C between 6 and 9°C, and then at
0.55/xm/day/°C between9and 13°C(Fig. 1). Landers (1976)
stated that A. islandica larvae will not survive metamorphosis
at 20°C. Lutz et al., (1982) did not examine the growth and
survival of A. islandica larvae above 15°C. If temperature
specific growth rate decreases linearly between 15 and 20°C
to a value of zero at 20°C, the decrease in rate with increas-
ing temperature would be 1.06 /xm/day/°C; however, Mann
and Wolf (1983, Fig. 4) indicated that the larva! swimming
temperature optima for A. islandica are in the range 1 5-1 8°C.
These latter data suggested that optimal growth may occur
between 15 and 18°G. Therefore, the model was run with two
different forms of temperature specific growth rate versus
temperature relationship (Fig. 1). In the first instance,
hereafter termed type A, temperature specific growth rate was
assumed to be constant between 1 3 and 1 5°C with a subse-
quent linear decrease in rate to zero at 20°C. In the second
form, type B, temperature specific growth rate was assumed
constant between 13 and 18°C with a subsequent decrease
to zero at 20°C.

Data describing optimum temperature for swimming
in Arctica islandica larvae were taken from Mann and Wolf
(1983, Fig. 4). Optimum temperatures for the lengths 90 /xm

B

MANN: DISTRIBUTION OF BIVALVE LARVAE

53

(trochophore stage), 110, 120, 145 and 204 pm were taken
as 17.0, 15.0, 17.0, 15.0 and 18.0°C. Optimum temperature
was assumed to be constant at 18°C for larvae between 204
and 265 fim length.

Data describing response to pressure changes were
taken from Mann and Wolf (1983, Figs. 2 and 3). AH
developmental stages showed a negative geotactic swim-
ming response that is enhanced under increased pressure
such that the preferred depth of occurrence up to 202
in length is in the range 0-5 m. The swimming response to
increased pressure and temperature optima of larvae in ex-
cess of 202 and 204 /*m length, respectively, was not ex-
amined. The assumption is made that the swimming response
of larvae in the length range 200-265 /im to both temperature
and increased pressure is identical to that of smaller larvae,
with the exception of the metamorphosing pediveligers which
are assumed to exhibit a positive geotaxis that overrides a
temperature optima response.

The growth model was not required to accommodate
for diurnal fluctuations in light intensity and spectral quality
because Arctica islandica larvae do not exhibit distinct
phototaxis (Mann and Wolf, 1983). The swimming behavior
of some other bivalve larvae is influenced by light (Mann,
1986).

The growth model used the four data sets (water
temperature-depth matrix, temperature specific growth rate,
optimum temperature and optimum pressure) to compute dai-
ly length increment and cumulative length at the prevailing
temperature of a fertilized egg originating at 45 m depth on
any specified day between April 13, 1981 (day 103 of the year
and the day of first sample collection in the field program)
and December 14, 1981 (day 346 of the year and the last sam-
ple date of 1981). After calculation of a daily length increment
for a day, d, the temperature for the following increment, that
is day (d + I), was made by selecting the appropriate value
from the water temperature-depth matrix to correspond with
the optimum temperature- optimum pressure combination for
the larval size as determined by the appropriate inputs. The
mode! therefore assumes active depth regulation by the
developing larvae. Where temperature and pressure optima
were not in agreement, that is an optimum temperature oc-

curring at depths greater than 5 m, the selection of optimum
temperature was made in preference to that of optimum
pressure, because the data of Landers (1976) suggested that
prolonged exposure to temperatures of only slightly above
the optima recorded here would be lethal. The depth value
chosen from such considerations of temperature and
pressure optima is a single deterministic value. In effect, it
is actually the upper (shallow) limit of a vertical distribution
range. Thus, the model never allowed larvae to experience
water temperatures in excess of their optimum temperature.
Results of the model consisted of a summary of daily length
increments, cumulative length, a record of depth occurrence
and accompanying temperature throughout development,
and an estimate of days required to attain metamorphic
length.

The sensitivity of the model to changes in shape of
the growth curve was tested (type A and B as given in Fig.
1 ). Sensitivity to changes in the temperature optima was also
examined by sequentially increasing the values from Mann
and Wolf (1983, Fig. 4) by one degree increments up to a
maximum value of 19°C.

Both Jones (1981) and Mann (1982) have described
the seasonal gonadal cycle of A. islandica for offshore New
Jersey and southern New England Shelf populations, in both
locations spawning appears to proceed from May through Oc-
tober with partly spawned individuals being most abundant
in August-October; however, the essentially descriptive nature
of these data make them somewhat intractable for quan-
titative modelling.

RESULTS

Table 1 summarizes field data as estimated numbers
of Arctica islandica larvae of > 200 length, which were
present at specific depths on the Southern New England Shelf
from April 13 through December 14, 1981 (Julian days
113-348). Note that these data deal predominantly with lar-
vae of > 200 /*m length, whereas model data are for a varie-
ty of lengths. The assumption is made that the responses ex-
hibited by the size classes of larvae examined are generally
representative of larvae throughout development. Arctica

Table 1. Numbers of Arctica islandica larvae of > 200 length m“3 collected at specific depths on the Southern New England Shelf during
the period April-Decernber 1981. ns: no sample collected due to net failure.

Date (1981)
Julian Day Number
Depth (m)

4/13

103

5/11

131

6/8

159

6/29

180

7/13

194

7/27

208

8/10

221

8/24

236

9/8

251

9/21

264

10/5

278

10/26

299

11/19

323

12/14

348

1

0

16

0

0

0

0

0

0

0

0

0

0

0

0

10

0

70

0

0

0

0

0

0

0

0

0

n.s.

5.4

0

20

0

1.5

0

0

0

0

0

0

273

18.7

0

n.s.

101*

0

30

0

0.1

0

0

0

0

0

7

512

41.3

187

n.s.

311*

0

40

0

0

0

0

0.2

0.3

1.9

0

33

6.5

138

n.s.

59*

0

All depths

0

88

0

0

0.2

0.3

1.9

7

818

66

325

0

5.4

0

indicates identification of first shelled larvae at length of 110 «m (modified from Mann, in press).

54

LARVAL ENTRAINMENT

* * * ******** * * *

APR MAY JUN JUL AUG SEP OCT NOV DEC

DAY NUMBER

* * * ******** * * *

APR MAY JUN JUL AUG SEP OCT NOV DEC

DAY NUMBER

Fig. 2. Depth-time isotherm diagram of temperature structure during the period April-December 1 981 based on vertical profiles with a sampl-
ing interval of 5 m. Heavy, curved lines labelled with even number values from 6 to 20 represent temperature in °C of isotherm. Superimposed
on the isotherm diagram are light straight lines representing predicted depth of occurrence oiArctica islandica larvae originating from spawn-
ings occurring on Julian days 113, 153, 183, 203, 223, 253 and 283. Where light lines terminate, arrows indicate predicted day of metamor-
phosis. Fig. 2A is model output for type A temperature specific growth rate input (see Fig. 1), Fig. 2B is for type B input (see Fig. 1). * Denotes
dates on which field observations of temperature and larval abundance were made.

islandica larvae occur in significant concentrations in May (1
and 1 0 m) and from September to November (20-40 m). Salini-
ty variation through the depth of the water column did not
exceed 0.15 °/00 on any one date; however, changes in both
absolute temperature and temperature stratification were evi-
dent during the study (Fig. 2). The May occurrence of A. islan-

dica larvae coincided with temperatures of 9-1 0°C prior to
thermal stratification. The 20-40 m depths during September
corresponded to 1 5-1 8°C water which is overlayed by warmer
water. During October and November temperature
destratification occurred and the water column mixed from
top to bottom. Water temperature decreases to 10°C by the

MANN: DISTRIBUTION OF BIVALVE LARVAE

55

end of November.

Predicted shallow limit of depth of occurrence
throughout development of growth types A and B larvae
originating from spawnings on specific dates during day
numbers 1 1 3 (April 1 3) through 283 (October 1 0) of the year
are illustrated in Figs. 2A and 2B. Irrespective of growth type,
larvae from spawnings prior to day 1 1 3 (April 1 3) experience
low temperatures, remain in the depth range of 0-5 m
throughout development, and reach metamorphic size,
together with those spawned on day 113, and on day 153
(June 2). Larvae originating from spawnings on day 1 53 swim
to the surface but soon encounter surface temperatures which
are sufficiently high to encourage depth regulation in the
range 5 to 15 m before metamorphic size is attained on day
1 92 (July 1 1 ) (type A) or 1 86 (July 5) (type B). The increasing
surface temperature during days 180-210 (June 29-July 29)
and the maintenance above 18°C until day 265 (September
22) restricts larvae spawned during days 173-213 (June
22-August 2) (type A) or 173-233 (June 22-August 22) (type
B) to depths greater than or equal to 10 m. Larvae originating
from spawnings on day 223 (August 12) rise to the depth
range of 0-5 m during the final 7 days of development in type
A growth. In type B growth larvae spawned on day 243
(September 1) rise to the surface for the final 17 days of
development. Although surface temperatures decrease slowly
after day 220 (August 9) the 1 7°C isotherms increase in depth
during days 220-265 (August 9-September 22). Larvae
originating from spawnings on day 223 (August 12) encounter
these “sinking” isotherms and remain at depths below them
during early development. Larvae originating from spawnings
on day 253 (September 1 0) experience a similar depth limita-
tion during early development; however, by day 283 (October
10) mixing of the water column has reduced thermal stratifica-
tion to a point where no barriers to vertical movement of any
of the developmental stages exist.

Examination of the sensitivity of this data in relation
to increasing temperature optima suggests that larvae
spawned in the early part of the year remain in or near the
surface waters slightly longer and return to them slightly
sooner following the thermocline decay. The suggested depth
range of occurrence of larvae during the mid year period still
“sinks” below the 19°C isotherm but remains above the 15°C
isotherm.

Figure 3 illustrates the relationship of predicted time
in days to attain metamorphic length versus day of spawn-
ing for spawnings originating at ten day intervals during the
period day 103-303 (April 1 3-October 30) under growth types
A and B. The type A growth-temperature relationship predicts
two periods when growth rate is high and metamorphosis is
reached in less than 35 days. These periods coincide with
spawnings on days 163 (June 12) and 263-283 (September
20-October 10). Prior to day 163 (June 12) growth rate is low
due to low temperature whereas between days 1 63 and 263
growth rate is again low due to temperatures in excess of
15°C. Decreasing water temperatures throughout the water
column in late October-December result in increasing time
to metamorphosis and is associated with spawnings after day
283 (October 10). Spawnings on or after day 303 encounter

I I | l I i (

0 100 300

DAY NUMBER

! I I I I | I I I I I I I

JFMAM JJASONO

MONTH

Fig. 3. Predicted time to metamorphosis (days) under growth types
A (O) and B (•) of Arctica islandica larvae originating from spawn-
ings which occur at 10-day intervals during the period day 103-303
versus day of spawning.

temperatures of 6°C or below during larval development and
fail to metamorphose. When the type B growth-temperature
relationship is used a time to metamorphosis of 33 days is
predicted for all spawnings from day 143 (May 23) to day 273
(October 1).

Increasing temperature optima resulted in greater
depression of growth rates during the mid summer period with
growth type A, but only marginally depressed growth rate dur-
ing days 190-220 (July 9-August 9) with growth type B when
temperature optima were fixed at 19°C.

DISCUSSION

The extent to which active depth regulation contributes
to the observed variation in distribution of Arctica islandica
larvae in the field can, in part, be estimated from the degree
of agreement of the model output and actual observations.
There is good agreement between the field data (Table 1)
and the predictive model output (Figs. 2A and 2B) on the
absence of A. islandica larvae in the depth range 0-1 0 m be-
tween days 180 (June 29) and 270 (September 27). The model
predicts aggregation of larvae at 10-25 m during days 180-213
(June 29-August 3), however, very small numbers are seen
in the field at this time. The model output agreed well with
field records of the presence of A. islandica larvae of > 200
/*m length throughout the 20-40 m depth range from days 194
to 278 (July 1 3-October 5). In contrast the prediction of ag-
gregation of A. islandica larvae in the 0-5 m depth range
following the breakdown of the seasonal thermocline was not
substantiated in that field data (Table 1) indicated both large
( > 200 /iim length) and small ( < 1 50 length) A. islandica
larvae at 20-40 m depth in November (day 323). Clearly, the
negative geotactic behavior of A. islandica larvae observed
in the unstratified water column of laboratory containers by
Mann and Wolf (1 983) is not the predominant force influenc-
ing vertical distribution in the field during the fall period when
there is active mixing throughout the depth of the water
column.

56

LARVAL ENTRAINMENT

Further discrepancies between model results and field
data are probably due to factors which are not represented
in the model. These include larval mortality rate from starva-
tion and/or predation in the field, larval loss from advection
and, as mentioned earlier, variation in spawning intensity.
Larval survival is probably inversely related to the time re-
quired to reach metamorphosis. A combination of less than
maximal spawning activity and, if type A growth is inferred,
greater than minimum predicted time to metamorphosis
would contribute to an explanation of lack of large larvae in
the field before day 236 (August 24). Such reasoning also
suggests that the occurrence of highest concentrations of lar-
vae will coincide with highest spawning activity (probably in
August-October) and shortest time to metamorphosis. Ir-
respective of whether growth type A or B is inferred, applica-
tion of this reasoning suggests the occurrence of highest lar-
val concentration between day 243 (August 31 ) and 303 (Octo-
ber 30). The peak of abundance of large A. islandica larvae
recorded in the field occurred between days 251 and 278
(September 8 and October 5 - see Table 1 and Fig. 2B). Net
failure on day 299 (October 26) prevents further statements
as to the end of this period of abundance of > 200 /x m lar-
vae; however, the presence of considerable numbers of small
(< 150 /jm) veliger larvae on day 323 (November 19 - see
Table 1) suggests that it may continue past day 278 (October
5). The model predicts that the large numbers of small larvae
present on day 323 (November 19) will not metamorphose
due to decreasing temperature. It does not examine the op-
tion suggested and discussed by Mann (in review) that these
larvae form the basis of an over-wintering population that
ultimately gives rise to large ( > 200 /xm) larvae in May of the
following year.

Despite the limitations of the relatively small data set
used to construct the present model, a reasonably good
agreement is seen between model results and field data. The
model is simplistic in that it is only two dimensional and does
not include an advective component; however, this develop-
ment is hindered by a lack of current data (temporally, spatial-
ly and with depth) for the study site (R. C. Beardsley, per-
sonal communication). The general application of the model
to other sites in the Middle Atlantic Bight is probably not
unreasonable in that the spawning stock of A. islandica is
widespread (Merrill and Ropes, 1969; Franz and Merrill, 1980;
Theroux and Wigley, 1983) and roughly synchronous in time
and intensity of spawning activity (Jones, 1981; Mann, 1982).
Additionally, the thermal structure, especially the develop-
ment and decay of the seasonal thermocline, is a relatively
constant conservative feature throughout the Southern New
England Shelf and Middle Atlantic Bight regions (Bigelow,
1933; Beardsley et al., 1976; Williams and Godshall, 1977;
Beardsley and Boicourt, 1981).

The comparison of field collected and laboratory
generated data sets presented here utilizes ony one common
data set, the time versus depth temperature matrix. In all other
respects the data are independent of one another. The
periods of agreement in the two final data sets strongly sug-
gest that active depth regulation occurs in Arctica islandica
larvae; however, the periods of discrepancy indicate that ac-

tive depth regulation does not always predominate. The
physical regimes corresponding to the transition from active
depth regulation to passive movement are poorly understood,
yet of obvious value. From a theoretical standpoint certain
quantitative aspects are well documented. The relative posi-
tions of the ciliated velum and the valves combined with the
pattern of ciliar beating dictate that bivalve larvae can only
swim in vertically oriented helices. Rates of vertical excur-
sion (i.e. change in actual depth, not absolute velocity) range
from < 1 to 1 0 mm/sec depending upon size of the larva and
water temperature. Bivalve larvae are flattened elipsoids that
rarely exceed 300 /xm in greatest dimension and have specific
gravity of approximately 1.3. If the changes in velocities of
water movement over discrete distances (be this centimeters,
meters or kilometers) were known, the application of a pure-
ly mathematical approach to the problem of active versus
passive processes in depth regulation and dispersal might
be possible. This option becomes even more pressing when
estuarine systems and other bivalve species are considered.
Estuaries exhibit salinity stratification with depth, a salinity
gradient along their length, and even the possibility of salini-
ty gradients across their width. Salinity stratification is minimal
on the New England Shelf. Estuarine circulation and stratifica-
tion are influenced by tidal, neap-spring tidal and gravitational
flow. Clearly, the physics of estuarine circulation are more
complex and dynamic than those of the shelf system de-
scribed here. Furthermore, larvae of estuarine bivalve species
may exhibit a far less conservative repertoire of behavioural
responses to environmental stimuli than do A. islandica lar-
vae. For example, the larvae of Ostrea edulis L. (Cragg, 1980)
and Mytilus edulis L. (Bayne, 1964) exhibit distinct phototaxis.
Nonetheless the present study demonstrates for bivalve lar-
vae, just as Sulkin and Van Fleuklem (1982) and Kelly et al.,
(1982) have done for blue and red crab larvae respectively,
the power of laboratory generated data and numerical models
in elucidating factors controlling distribution and dispersal in
stratified coastal systems. The application of such models
to estuarine species is clearly warranted.

ACKNOWLEDGMENTS

This work was supported by U.S. Department of Commerce,
N.O.A.A., Sea Grant under Grant Number NA80-AA-D-00077, Office
of Naval Research contract N00014-79-C-0071 NR 083-004 and the
Andrew Mellon Foundation.

LITERATURE CITED

Bayne, B. L. 1964. The responses of the larvae of Mytilus edulis L.

to light and to gravity. Oikos 15:162-174.

Beardsley, R. C. and W. C. Boicourt. 1981 . On estuarine and Con-
tinental Shelf circulation in the Middle Atlantic Bight. In: Evolu-
tion in Physical Oceanography: Essays on the 60th Birthday
of Henry Stommel. B. A. Warren, C. S. Wunsch, eds., pp.
198-233. MIT Press, Cambridge, MA, USA.

Beardsley, R. C., W. C. Boicourt, and D. V. Hansen. 1976. Physical
oceanography of the Middle Atlantic Bight. In: Middle Atlan-
tic Continental Shelf and the New York Bight, Vol. 2, M. G.
Gross, ed., pp. 20-34. American Society of Limnology and
Oceanography, Lawrence, Kansas.

MANN: DISTRIBUTION OF BIVALVE LARVAE

57

Bigelow, H. B. 1933. Studies of the waters on the continental shelf,
Cape Code to Chesapeake Bay, I, The cycle of temperature.
Papers on Physical Oceanography and Meterology 2(4): 1-135.

Cragg, S. M. 1980. Swimming behavior of the larvae of Pecten max-
imus (L). Journal of the Marine Biological Association of the
United Kingdom 60:551-564.

Drinnan, R. E. and W. B. Stallworthy. 1979. Oyster larval populations
and assessment of spatfall, Bideford River, P.E.I., 1961.
Fisheries and Environment Canada. Fisheries Marine Service
Technical Report No. 795, 13 pp.

Fogarty, M. J. 1981. Distribution and relative abundance of the Ocean
Quahog Arctica islandica in Rhode Island Sound and off Mar-
tha’s Vineyard, Massachusetts. Journal of Shellfish Research
1(1):33-40.

Franz, D. R. and A. S. Merrill. 1980. Molluscan distribution patterns
on the continental shelf of the Middle Atlantic Bight (Northwest
Atlantic). Malacologia 19(2):209-225.

Helm, M. M. and P. F. Millican. 1977. Experiments in the hatchery
rearing of Pacific oyster larvae ( Crassostrea gigas Thunberg).
Aquaculture 11:1-12.

Jones, D. S. 1981. Reproductive cycles of the Atlantic surf clam
Spisula solidissima, and the ocean quahog Arctica islandica
off New Jersey. Journal of Shellfish Research 1(1):23-52.

Kelly, P., S. D. Sulkin and W. F. Van Heuklem. 1982. A dispersal
mode! for larvae of the deep sea red crab Geryon quinquedens
based upon behavioral regulation of vertical migration in the
hatching stage. Marine Biology 72:35-43.

Landers, W. S. 1976. Reproduction and early development of the
Ocean Quahog, Arctica islandica , in the laboratory. Nautilus
90:88-92.

Lutz, R. A., R. Mann, J. G. Goodsell and M. Castagna. 1982. Larval
and early post larval development of the Ocean Quahog Arc-
tica islandica. Journal Marine Biological Association United
Kingdom 62:745-769.

Mann, R. 1982. The seasonal cycle of gonadal development in Arc-
tica islandica from the southern New England Shelf. Fishery
Bulletin 80(2):31 5-326.

Mann, R. (1986). Sampling of bivalve larvae. In: North Pacific
Workshop on stock assessment and management of in-
vertebrates. G. S. Jamieson and N. Bourne, eds., (in press)
Canadian Special Publication in Fisheries and Aquatic Science
92.

Mann, R. (in press) Seasonal changes in the depth distribution of
bivalve larvae on the Southern New England Shelf. Journal
of Shellfish Research.

Mann, R. and C. C. Wolf. 1983. Swimming behavior of larvae of the
ocean quahog Arctica islandica in response to pressure and
temperature. Marine Ecology Progress Series 13:211-218.

Merrill, A. S. and J. W. Ropes. 1969. The general distribution of the
Surf Clam and Ocean Quahog. Proceedings of the National
Shellfisheries Association 59:40-45.

Ropes, J. W. 1978. Biology and distribution of surf clams (Spisula
solidissima) and ocean quahogs (Arctica islandica) off the north-
east coast of the United States. In: Proceedings Northeast
Clam Industry Management for the Future. Sea Grant Program;
University of Massachusetts and Massachusetts Institute
Technology. Report number 112:47-66.

Sulkin, S. D. and W. Van Heuklem. 1982. Larval recruitment in the
crab Callinectes sapidus Rathbun: An amendment to the con-
cept of larval retention in estuaries. In: Estuarine Comparisons,
V. S. Kennedy, ed., pp. 459-476. Academic Press, NY.

Theroux, R. B. and R. L. Wigley. 1983. Distribution and abundance
of east coast bivalve mollusks based on specimens in the Na-
tional Marine Fisheries Service Woods Hole collection. NOAA
Technical Report number NMFS SSRF-768, 172 pp.

Walne, P. R. 1965. Observations on the influence of food supply and
temperature on the feeding and growth of the larvae of Ostrea
edulis L. Fishery Investigations, London. Series 2.24(1):1-44.

Williams, R. G. and P. A. Godshall. 1977. Summarization and inter-
pretation of historical physical oceanographic and
meteorological information for the Mid Atlantic Region. NOAA
Center for Experimental Design and Data Analysis, Final
Report to Bureau of Land Management Oct. 1977, 306 pp.

A REVIEW OF SOME FACTORS THAT LIMIT OYSTER
RECRUITMENT IN CHESAPEAKE BAY

GEORGE R. ABBE

THE ACADEMY OF NATURAL SCIENCES
BENEDICT ESTUARINE RESEARCH LABORATORY
BENEDICT, MARYLAND 20612, U.S.A.

ABSTRACT

From 1966 to 1979, oyster [Crassostrea virginica (Gmelin)] recruitment in the Maryland
Chesapeake Bay was poor during 10 years and only fair during the other 4. Although adults may undergo
normal gonadal maturation and spawn successfully, recruitment of setting larvae can still fail. Several
factors can increase the time required for eggs to develop into ready-to-set pediveliger larvae. This
time is normally 2 to 3 weeks, but as time increases, success rate for setting and subsequent spat
growth rate decreases. Even after rapid maturation to the pediveliger stage, setting can be limited
by inadequate substratum, strong currents, predators, pollutants, or other poor physical conditions.
Following successful setting, spat densities can be reduced by predation, burial during storms, silt
deposition, or overgrowth by competitors. Of major importance to successful recruitment is presence
of adequate broodstock, retention of larvae in the estuary until they are old enough to set, and availability
of clean hard substrate that can be augmented by planted shell material.

The American oyster Crassostrea virginica (Gmelin)
has traditionally been the most valuable shellfish in the
Maryland Chesapeake Bay, even when landings were low.
Average annual production during 1976-82 equalled only
about 101,000 m3 (2.2 x 106 Maryland bushels; a Maryland
bushel equals 0.0459 m3 [2800.7 in3]) of oysters, an amount
far below what the region once produced. Although landings
of 688,000 m3 (15 x 10® bushels) as in 1885 (Kennedy and
Breisch, 1981) will probably never be seen again, 184,000
to 230,000 m3 (4 to 5 x 10® bushels) is not an unrealistic goal.
Harvests during the 1983-84 and 1984-85 seasons were about
46,000 m3 (1 x 1 0® bushels), and the outlook for the 1 985-86
season was just slightly better (W. Outten, Maryland Depart-
ment of Natural Resources, Annapolis, Md., pers. comm.).

One of the main causes of reduced oyster harvests
has been and continues to be the failure over a multi-year
period of larvae to survive either the planktonic stage of
development or the subsequent metamorphosis or first few
days postmetamorphosis (see Meritt, 1977). In a model
developed to help estimate future oyster harvests in Maryland,
Ulanowicz et at. (1980) showed that setting of oyster spat
varied directly with the cumulative high salinity during the
spawning season and inversely with the harvest during the
previous season. From 1966 to 1979 there were 10 years of
very poor recruitment in Maryland (Baywide average) and 4
years when it was only fair (Krantz et al., 1982).

In view of the decline of the Maryland oyster fishery

to a record low level, it is time to examine factors that limit
success of oyster recruitment in Chesapeake Bay. Perhaps
this information will aid efforts to control some of these fac-
tors and reverse the decline in oyster harvests. This paper
reviews the more important or obvious factors instrumental
in limiting the success of recruiting oysters.

A number of factors can result in poor oyster recruit-
ment during a reproductive season. One is the failure of adults
to spawn, which most probably results from prolonged low
salinity or sudden salinity reduction caused by spring rains
and flooding (May, 1972; Cake, 1983a). Loosanoff (1953)
demonstrated the effects of low salinity on gametogenesis
and spawning of adult oysters from Long Island Sound. He
found that gametogenesis was completely inhibited in fresh
water and at a salinity of 3 °/0o. At 5 °/00 about half the oysters
produced gametes, but at lower rates than at higher salinities.
At 7.5 °/0o some gametes ripened, but more slowly than at
10 °/ oo and higher where development and maturation
proceeded normally. Normal development was slowed or
stopped when salinity fell below 7.5 °/00. Fully ripe oysters
spawned at 5 0/00> but spawning was weak; at 7.5 °/00 and
above spawning appeared normal. Rapidly declining or low
salinity can therefore affect the gonads at all stages of
development, and once stopped, gonadal activity may not
reach normal levels for 3 to 4 months (Butler, 1949).

For purposes of this review I will assume that adults
have spawned and that trochophore and veliger larvae are

American Malacologicai Bulletin, Special Edition No. 3(1986):59-70

59

60

LARVAL ENTRAINMENT

Fig. 1. Location of the Calvert Cliffs Nuclear Power Plant and the
adjacent Flag Pond Oyster Bar.

present in the water column, although in reality the act of
spawning does not preclude the presence of larvae. Other
factors then become critical to the success or failure of recruit-
ment. These are (a) retention of healthy larvae up to
pediveliger stage within the estuary where they can set and
maintain the population and (b) availability of clean
substratum to which pediveligers can attach. These factors
are discussed in the sections below.

Supporting evidence is based on observations and
results of field studies conducted at or near the Calvert Cliffs
Nuclear Power Plant (CCNPP) on the western shore of
Chesapeake Bay in Calvert County, Maryland, from 1979 to
1 985 by the author (Abbe, 1981; 1 984; 1 985). These studies
employed divers using SCUBA to gather data or source
material. In one study densities were determined by coun-
ting the legal-size (^76 mm), sublegal, and spat oysters on
both sides of the upper, middle, and lower thirds of five ran-
domly selected panel sections (each 12 m2 per side) of the
intake embayment curtain wall of the CCNPP (Fig. 1).

In the second study densities of three size classes
were estimated at various locations on the Flag Pond Oyster
Bar (Fig. 2). A random guadrat design similar to that used
by May (1971) in his survey of the Alabama oyster resources

was used (described in detail by Abbe, 1984). Oysters and
associated bottom material were collected by divers from
within square steel frames (each 0.33 m2) dropped to the bot-
tom from the research vessel. Material was brought to the
surface where it was examined and counted.

Larvae generally mature in 2 to 3 weeks, but this time
can be lengthened by marginal growing conditions. As dura-
tion of larvae in the water column increases, to 4 weeks or
more, success rate of metamorphosis and setting decreases.
Losee (1979) showed a direct relationship between time for
larval development and subsequent growth of spat, with
shorter development time leading to more rapid post-
metamorphic growth rates during 12- and 29-week periods.
Whether this advantage continues for longer periods remains
to be determined. Newkirk (1981) showed a slight correla-
tion between larval growth rate and juvenile size for Ostrea
edulis (linne) also, but the effect of larval growth rate de-
creased over time until it approached zero when the oysters
averaged 43 mm in shell height. The effect on long-term
growth rate is immaterial, however, provided increased spat
growth gets it past the critical early postmetamorphic stage
when mortalities typically range from 75% to 95% or more
in a hatchery (Kranz, 1982). Below are several factors that
can increase the time required for larvae to grow from egg
to ready-to-set pediveligers.

INCREASED LARVAL DEVELOPMENT TIME

LOW TEMPERATURE. Larvae entrained in warm
water that mixes with cooler water will slow their rate of
development. While oysters from northerly areas, such as
Long Island Sound, can spawn at cooler temperatures than
oysters from Delaware, Maryland and more southerly areas

Fig. 2. Locations of selected study areas within Flag Pond Bar.

ABBE: FACTORS LIMITING OYSTER RECRUITMENT

61

(Loosanoff, 1969; Price and Maurer, 1971; Kennedy and
Breisch, 1981), normal development continues from the
temperature at which eggs are fertilized. If spawning occurs
earlier than normal in an unusually warm area (e.g., in shallow
creeks or bays that are warmed by the sun or in areas in-
fluenced by industrial thermal effluents), and larvae are then
carried into colder water, their development will be slowed.
Ruddy et at., (1975) demonstrated that oysters held in a
heated power plant effluent developed gonads up to 4 months
earlier than controls and spawned up to 1 month earlier,
although this case may not be representative of other areas.

HIGH TEMPERATURE. Larvae may hatch during the
normal spawning season and then be entrained in waters that
are too warm for normal development, such as produced by
some electrical generating stations and other industrial
facilities. Larvae generally occur when water temperatures
are in the low to upper 20s (Davis and Calabrese, 1964; Hidu
etai, 1974), although Kennedy and Krantz(1982) observed
oysters spawning in Chesapeake Bay at temperatures between
15 and 20°C. They postulated that spawning may be initiated
by some food-related chemical or temperature-food stimulus
for oysters in warmer temperate waters rather than by
temperature alone. Larvae develop at temperatures up to 30
or 32°C (Davis and Calabrese, 1964; Hidu et at., 1974), but
a higher temperature can be tolerated for short periods. The
younger the embryo or larva the more easily it is damaged
by high temperatures. If temperatures are elevated only a few
degrees, but remain within the acceptable developmental
range, larval time can be decreased rather than increased
since warmer temperatures result in higher growth rates
(Medcof, 1939; Davis and Calabrese, 1964; Kennedy and
Breisch, 1981).

SALINITY. Larvae generally do well at salinities at
which gonads of parent oysters mature. Optimum salinity for
larvae of parents held at 26-27 °/00 was 17.5 °/00, but de-
creased to 12.5 °/00 for larvae of parents held at 9 °/00 (Davis,
1958). As salinity declines below an optimal level, the larval
maturation time increases and survival rate decreases. Davis
(1 958) showed that 1 65-/xm larvae from parents held at 26-27
°/00 grew well at salinities as low as 12.5 °/0 0, but at 10 °/00
and 7.5 °/00 their growth was only a half and a quarter, respec-
tively, that of control growth. Similar effects of low salinity
were demonstrated for larvae of Ostrea edulis Linne (Davis
and Ansell, 1962). A salinity reduction from 26 °/0o to 15 °/00
had no effect on growth, but growth was inhibited below 1 2.5
°/ oo- At 10 °/0o, 90 to 95% of the larvae died in 2 weeks and
at lower salinities death was even sooner.

LOW DISSOLVED OXYGEN (DO). Adult oysters can
survive for several days when DO concentrations are less
than 1.0 mg 02-P (Sparks etai, 1958), but survival time varies
inversely with temperature. Dunnington (1968) showed that
buried adult oysters survive only 2 days in summer, but up
to 5 weeks in winter. Because resistance of larvae to stress
is less than that of adults, larvae probably cannot tolerate low
DO concentrations for long either and can be killed quickly
by anoxic conditions. Much of the deeper portion of
Chesapeake Bay (below 10 m) has summertime DO levels
of 0 to 4 mg 02-/'1, and it was estimated that the volume of

water with DO concentrations equal or less than 0.7 mg 02n
was 15 times greater in 1980 than in 1950 (Taft et at., 1980;
Mackiernan et at., 1983; Officer et at., 1984). Because lar-
vae are present during the warmest time of the year when
DO levels are normally lowest, upwelling can result in DO
concentrations in shallow water that rapidly reach critical
levels (0 to 2 mg O2-/'1) that probably retard development or
kill larvae caught in this hypoxic or anoxic water. Toxic H2S
will form at 0.0 mg 02 -P concentration.

SUSPENDED SEDIMENTS. Davis and Hidu (1969a)
determined that as little as 188 mg /'1 of silt caused a signifi-
cant reduction in the percentage of oyster eggs developing
to straight-hinge stage, and at 500 mg /'1 less than a third
developed to straight-hinge. A silt level of 750 mg /'1 caused
a significant reduction in growth and decrease in survival of
larvae. At 2 g-/'1 no growth occurred and at 3 g /'1 all larvae
eventually died. Levels of 2 to 3 g/'1, however, seldom occur
naturally over oyster beds. Davis and Hidu (1969a) postulated
that the adverse effects could have been caused directly by
particulates or by reduced pH (which sometimes fell from 7.5
to 6.4) caused by an increased silt load. Calabrese and Davis
(1966) showed that growth rates of larvae drop rapidly at pH
levels below 6.75 and that the lower limit for survival is 6.0.
Davis and Hidu (1969a) concluded that larvae can tolerate
levels of suspended sediments higher than normally found
in natural waters, but that effects of dredging or filling can
produce particulate levels that are detrimental to larvae.

LACK OF PROPER FOOD. Investigations of feeding
and nutritional requirements of oysters show considerable
differences among larvae, juveniles, and adults, with larvae
being more restricted as to algal species composition and
cell size than adults (Davis, 1953; Ukeles, 1971 ; Fritz et a!.,
1984). Different stages of larval development select different
sizes of food particles (Mackie, 1969). She showed that
straight-hinge larvae select phytoplankton in the 1 to 10
size range, while early and late umbo stages take particles
up to 18 /xm, and pediveligers feed on cells up to 30 ^m.
Sublethal concentrations of certain pollutants may destroy
one of the critical links in the larval food chain that could then
be replaced by a bloom of microorganisms such as Chlorella
sp. or toxic dinoflagellates totally unsuitable for larvae
(Guillard, 1958; Galtsoff, 1964).

POLLUTANTS. Extremely low concentrations of some
pesticides, herbicides, chlorine-produced oxidants (CPO),
detergents, heavy metals, and petroleum hydrocarbons can
retard growth or cause mortality of eggs and larvae of C.
virginica. A concise review of the effects of these chemicals
is included in Kennedy and Breisch (1981). Davis (1961) found
a wide range of toxicity among various insecticides, her-
bicides, oils and organic solvents, and antibiotics and disinfec-
tants to larvae. A decrease in the growth rate was the first
sign of toxicity; however, only after growth rates were slowed
by at least 50% did significant mortality occur. Calabrese and
Davis (1967) determined that the percentage of eggs that
developed normally was reduced significantly at concentra-
tions greater than 0.025 mg /'1 of active linear alkylate
sulfonate detergent and 0.25 mg-/'1 commercial liquid
biodegradable detergent. Survival decreased at levels of 1 .00

62

LARVAL ENTRAINMENT

and 2.50 mg /'1 for the two detergents, respectively.
Calabrese et al., (1977) found that LC5 concentrations of Hg,
Ag, Cu and Ni, did not affect growth, but growth was reduced
at LC50 concentrations. Zaroogian and Morrison (1981)
showed that while seawater concentrations of Cd as low as
5 jtg-kg"1 may not be toxic to embryos, these concentrations
could cause enough stress to delay development to larvae.
This could prolong the pelagic stage and ultimately result in
reduced recruitment into the population. Chlorine-produced
oxidants have traditionally been used by industry to prevent
biofouling of cooling water systems and by sewage treatment
plants to reduce bacterial contamination of receiving waters.
Chlorine-produced oxidants, unfortunately, are toxic to many
forms of aquatic life including oyster larvae (Roosenburg et
al., 1980; Richardson et al., 1982).

New compounds are continually being developed and
many of these are put into use before their toxicity to aquatic
organisms has been thoroughly tested. Thus little is known
of the effects these compounds have on aquatic organisms
when they are inadvertently or intentionally introduced into
rivers and estuaries. Other old pollutants can remain buried
in the bottom awaiting release and resuspension by dredg-
ing or by natural water currents. The task of determining the
effects of even some of these chemicals in the natural en-
vironment becomes monumental when one considers possi-
ble synergistic effects and their relationship to larval de-
velopmental stages and physical conditions (Kennedy and
Breisch, 1981). This task grows even larger when effects of
changing temperature, salinity, and DO are included.

PREVENTION OF LARVAL SETTLING

Assuming that environmental conditions are favorable
and larvae have matured rapidly to the pediveliger stage,
several factors can still interfere with successful setting.

The first of these is the substratum. Pediveligers can
attach to a wide variety of solid materials including shells,
rocks, cement, wood, metal, glass, rope, rubber, and plastic
(Galtsoff, 1964) provided the surface is relatively clean (not
oily, fouled by invertebrates, or covered with sediment). Lar-
vae, however, do not recruit to soft mud and sand bottoms
that are unstable, but mixtures of mud and sand are generally
stable enough to provide support for oyster communities. Firm
bottoms covered with cultch of rocks, shells, or other material
can be rendered unsuitable for spat settlement by various
invertebrates occupying the space. Fouling can be controlled
to some extent by use of granulated calcium oxide spread

over oyster beds at a rate of 6.75 metric tons ha'1 to kill fouling
organisms (Galtsoff, 1964; MacKenzie, 1977). Rock and shell
bottoms can also be rendered unsuitable by a layer of sedi-
ment; 1 to 2 mm of loose sediment on potential cultch may
prevent setting of mature larvae (Galtsoff, 1964). This state-
ment appears to be based on empirical rather than ex-
perimental data. If a 1- to 2-mm layer of sediment does pre-
vent setting, it is unclear whether the sediment serves as a
mechanical barrier to the larva, a barrier to recognition of shell
proteins, or release of a substratum-produced chemical that
induces settlement (Crisp, 1967; Hadfield, 1984). Sediments
can be resuspended by use of bagless oyster dredges or
pressure boards so that tidal currents will then carry the sedi-
ment away (MacKenzie, 1983), but this is costly and may not
achieve the desired results if sediments are redeposited near-
by. MacKenzie (1981) noted that in Long Island Sound silt
accumulates on various oyster beds to depths of 1 to 5 cm
during January to April, but that most of it is gone by May
or June. Clean hard substratum without excessive fouling or
silt is perhaps the single most necessary factor for successful
setting (Galtsoff, 1964).

The importance of substratum is illustrated by the shell
planting on Flag Pond Bar (Fig. 1) near Calvert Cliffs. Abbe
(1984) showed that oyster population size in that area was
essentially unchanged from 1968 to 1979 during which no
shells were planted and spat setting was light. In 1980 the
Maryland Department of Natural Resources planted 102,000
bushels of shells in Area 2 (Fig. 2), in 1982 another 197,000
bushels were planted in Areas 4 and 5, and in 1984 Areas
4 and 5 received another 70,000 bushels (W. Outten,
Maryland DNR, Annapolis, Md., pers. comm.). Spat setting
increased baywide during 1 980-82 over the previous decade
(Krantz and Davis, 1983) and was also excellent again dur-
ing 1985. The effect of planted shell on increased numbers
of sublegal oysters and spat during 1983-85 are evident in
Table 1. No shell material was ever placed on Area 6 (Fig.
2), and this is reflected in the light spatfall in 1985 (Table 1).

Many larvae are lost from setting areas as a result of
being carried seaward by tidal currents. Estuarine circula-
tion patterns described by Pritchard (1953) show in many
estuaries a net seaward movement of surface water and a
net flow up-estuary of bottom water. The seaward movement
of surface water would account for wide dispersal of young
larvae swimming in upper water layers and concentration of
late-stage larvae swimming near the bottom. The method of
retention of larvae within the estuary, however, has not yet
been fully explained. Some investigators support the theory

Table 1. Densities of oysters per m2 in 1979, 1983, 1984, and 1985 in four areas of Flag Pond Oyster Bar (spring and autumn combined).

Legal Sublegal Spat

1979

1983

1984

1985

1979

1983

1984

1985

1979

1983

1984

1985

Area

2

0.9

3.3

2.9

2.5

0.6

13.1

5.9

3.2

0.3

1.4

0.3

15.2

Area

4

2.6

5.6

6.8

6.6

0.8

45.0

11.8

6.7

0.2

2.4

1.8

43.2

Area

5

1.1

2.9

3.7

6.7

0.7

39.5

64.6

34.9

0.1

2.8

2.8

24.9

Area

6

-

4.8

4.8

5.7

-

8.2

3.7

1.9

-

0.2

0.1

2.4

Mean

1.7

4.3

4.7

5.4

0.7

27.6

20.1

13.2

0.2

1.8

1.1

23.4

ABBE: FACTORS LIMITING OYSTER RECRUITMENT

63

that larvae respond to salinity changes by remaining near the
bottom during ebb tide and rising in the water column dur-
ing flood tide, thus maintaining their horizontal position in the
estuary (Carriker, 1951; Kunkle, 1957; Wood and Hargis,
1971; MacKenzie, 1981; Haven and Fritz, 1985). In contrast,
others note that larvae swim continuously during larval life,
with their dispersal and fate dependent on currents and
flushing rates of estuaries (Andrews, 1983). When larvae sur-
vive to reach the umbo stage (3 to 5 days), they seek deeper
water and begin an up-estuary movement. Larval losses are
estimated at 15% per tidal cycle, but these are continuously
replaced by new broods where brood stock is sufficient (An-
drews, 1983). Losses of young larvae by dispersion, however,
are probably large regardless of the retention mechanism
used, but rate of loss probably declines as larvae mature and
seek deeper water that can carry them up-estuary (Andrews,
1985).

Enormous numbers of larvae are consumed by
predators, and many more larvae are trapped accidentally
on mucus-coated gill surfaces by filter-feeding organisms.
Ctenophores feed heavily on oyster larvae. Mnemiopsis leidyi
A. Agassiz, an unselective filter-feeding ctenophore, filters
4 to 100 /-day'1 and can consume nearly 500 copepods an
hour (Bishop, 1967). In the Patuxent River, Maryland, Bishop
found these rates equal to about 31 % of the copepod stand-
ing crop. At high densities M. leidyi can decimate the
zooplankton community (Mountford, 1980). Numbers of
ctenophores can be reduced when the scyphozoan medusa
Chrysaora quinquecirrha (Desor) is abundant (Mansueti,
1955; Mountford, 1980). This could result in reduced preda-
tion by ctenophores, but then C. quinquecirrha probably in-
gests substantial numbers of larvae as well (D. Cargo,
Chesapeake Biological Laboratory, Solomons, Md., pers.
comm.). Another voracious predator is the bay anemone
Diadumene leucolena (Verril) that MacKenzie (1977) deter-
mined is able to capture more than one larva per minute.
When densities of anemones number several hundred or
more per m2, they can effectively reduce the number of
pediveliger larvae available for setting.

Many other organisms trap pediveliger larvae on their
gill surfaces during respiration and feeding. If large particles
or larvae cannot be utilized for food, they are rejected (sorted
by the labial palps and discharged as pseudofeces by
oysters). MacKenzie (1 981 ) showed a 20% reduction in spat
per shell for shells located within 1 5 cm of 2-year-old oysters
compared to shells 1 m away (33.8 spat-shell'1 vs. 42.5
spat-shell'1). He also found that hard clams, dwarf surf clams,
blue mussels, slipper shells, barnacles and mud-tube worms
filtered and rejected larvae. MacKenzie concluded that lar-
vae rejected in loosely aggregated pseudofeces that were
broken up by currents escaped alive. If, however, they were
ingested and passed through the gut undigested, they were
unable to escape from compacted feces and were lost.

Salinity or DO concentrations can reach critically low
levels when pediveligers are ready to set, thus reducing their
ability to do so. Areas where salinity is already near the lower
limit for setting (10 to 12 °/0 0; Davis, 1958), an influx of fresh
water from rainfall can decrease salinity to levels that pre-

vent setting. Engle (1947) described conditions during 1944-46
when excessive rainfall prevented settlement in Maryland,
and Haven et al. (1977) discussed the impact of low salinity
from Tropical Storm Agnes in 1972 on oyster spat set (or lack
of it) in Virginia waters. Haven et al. (1977) suggested that
recruitment failure could also have been related to increased
silt and low DO levels. In the presence of hypoxic or anoxic
water in much of the Bay at depths below 9 to 10 m during
the summer (Taft et al., 1980; Officer et al., 1984), a day or
two of westerly winds can result in upwelling of this deep poor-
ly oxygenated water along the western shore of the Bay that
can be lethal to crabs and fish caught in crab pots (Abbe,
1 983). If upwelling of anoxic water occurs when pediveligers
are ready to set, they can be killed. Because they seek deeper
water at this time of their development (Wood and Hargis,
1971; Andrews, 1983), they could be at high risk.

Wave action and light are two factors that could affect
setting in shallow areas along the shore or on vertical sur-
faces. In a study of the oyster population on the intake em-
bayment curtain wall of the CCNPP in 1980, higher densities
of spat were found on the middle and lower thirds of the wall
than on the upper third (each third was 2.7 m deep) (Abbe,
1981). This depth difference could have been due to decreas-
ing light intensity with depth. Studies have shown that larvae
prefer darkened conditions when setting (Ritchie and Menzel,
1969; Shaw etal., 1970), though others have suggested that
light stimulates larvae to set (Medcof, 1955). For the upper
third of the wall, however, light intensity was similar on both
sides, yet spat density was 10.8 per m2 inside the embayment
wall, but only 4.2 per m2 on the bayside (Table 2). Ratios of
inside to bayside densities for each size class (Fig. 3) also
illustrate the greater difference between the two sides near
the surface than near the bottom. Although evidence is cir-
cumstantial, wave action against the outside of the wall may
account for some of these differences. The wall has several
openings in it during the summer (4.6 x 10.0 m corregated
steel panels are removed), so water is similar on both sides
as is the biofouling on the wall itself. Current patterns could
vary slightly; however, spat and sublegal oysters did not show
differences below 5.4 m exhibited at the surface (Table 2).
Wave action in this case appears to be the factor accounting
for such large differences.

Although pediveligers are more tolerant of toxic com-
pounds than developing embryos, they can be damaged or
killed by low levels of petroleum hydrocarbons (Renzoni,
1975; Wolfe, 1977), toxic metals (Calabrese etal., 1973; Cun-
ningham, 1979), pulp mill effluents (Galtsoff, 1964), chlorine-
or ozone-produced oxidants (Roosenburg et al., 1980;
Richardson et al., 1982), and agricultural runoff carrying
various pesticides and herbicides (Davis and Hidu, 1969b;
Calabrese, 1972; Kennedy and Breisch, 1981).

SPAT MORTALITY

There are no published studies of larval mortalities in
the field for Crassostrea virginica (Kennedy and Breisch,
1 981 ). Virginia Institute of Marine Science shellfish scientists

64

LARVAL ENTRAINMENT

Table 2. Legal, sublegal, and spat densitites (oysters per m2) at three depth ranges on the intake embay-
ment wall of the Calvert Cliffs Nuclear Power Plant in 1980.

inside Outside

Depth (m)

Legal

Sublegal

Spat

Legal

Sublegal

Spat

0.0-2. 7

5.0

0.4

10.8

1.0

0.1

4.2

2. 7-5. 4

10.4

1.5

18.2

3.9

0.6

11.7

5. 4-8. 2

12.0

1.8

13.4

5.9

2.0

14.2

M. Castagna and D. Haven hypothesized that larval survival
between the time of egg fertilization and spat setting is about
3.3 x 1 0"50/o or about one in three million (Lunz, 1 985). They
estimated that additional mortality between spat and seed
size could be as high as 90%, but less than the 99.8% first-
year mortality estimated by Nelson and Chestnut (1945) for
spat under extremely crowded conditions (98 spat per cm2).
Once set, spat can close their shells tightly and be less
susceptible than larvae to intermittent low salinity, low DO,
or pollutants. Because spat set in summer, however, they pro-
bably put much of their energy into growth and maintenance
rather than into glycogen storage, which doesn’t begin for
older oysters until spawning is completed in late summer
(Galtsoff, 1964). If spat do not store glycogen until autumn,
they would be less resistant to adverse conditions than larger
oysters with some reserve material. This question, however,
needs further study.

Newly set spat are subject to predation by oyster drills
(gastropods), sea stars, flatworms, crabs, fish, and birds. Drills
are perhaps the most serious of all oyster predators over the
entire range of C. virginica (Carriker, 1955; Galtsoff, 1964;
Cake 1983a, b). IJrosalpinx cinerea (Say) and Eupleura
caudata (Say) are the only drills found in the Chesapeake.
U. cinerea is more abundant in Maryland waters than E.
caudata because of its slightly lower salinity tolerance (18
0/00 compared to 20 °/00 for E. caudata ) (Lippson, 1973). These
salinity limits confine both drills to Maryland’s lower eastern
shore and prevent serious widespread damage to oyster
populations in Maryland, in the lower Chesapeake Bay,
however, and other high salinity areas of the Atlantic coast
from Canada to Florida, oyster drills are major predators (An-
drews, 1956; Galtsoff, 1964). Both species prey on oysters
by insertion of the probiscis through a small hole in the
oyster’s shell that they bore by alternating chemical and
mechanical activity (Carriker, 1955, 1961). Other predatory
gastropods found in the southern Chesapeake Bay include
Busycon spp. and Boonea ( = Odostomia) spp. (Kennedy and
Breisch, 1981; Lippson and Lippson, 1984). Busycon
canaliculatum Linne and Boonea impressa (Say) are occa-
sionally found well into the Maryland bay (Lippson and Lipp-
son, 1984), but damage to oyster populations by these gastro-
pods throughout the Bay is minor compared to that by drills.

The common sea star, Asterias forbesi (Desor) requires
salinities of 16 to 18 °/00 or higher (Galtsoff, 1964) and is
therefore not common in the Maryland portion of Chesapeake
Bay (Kennedy and Breisch, 1981). Although it is sometimes
abundant near the mouth of the Bay, it feeds primarily on
snails, mussels, clams, and barnacles (Lippson and Lippson,

1984), and does not cause the damage to oyster populations
that it does in New England waters (MacKenzie, 1970, 1981).

Unlike drills and sea stars, flatworms (Stylochus spp.)
tolerate a wide range of salinity and can also be highly
destructive of small oysters. Loosanoff (1956), however,
stated that they would readily attack oysters as large as 64
mm. Webster and Medcof (1961) presented circumstantial
evidence for oyster mortality caused by flatworms in the
Chesapeake, and Provenzano (1961) showed nearly 100%
mortality of newly set spat in ponds in Massachusetts at-
tributed to flatworms. Earlier evidence of flatworm predation
came from Pearse and Wharton (1938) who investigated
oyster mortalities in Florida and from Loosanoff (1956) who
reported mortalities resulting from flatworms in Connecticut.
Stylochus ellipticus (Girard) is widely distributed along the east
coast and is commonly associated with barnacle and oyster
communities (Hyman, 1940). It is the only flatworm that is
predatory on oysters in the Chesapeake, but the extent of
its damage to oysters in the field is unknown. Landers and
Rhodes (1970) demonstrated that S. ellipticus will not always
prey on oysters. In fact, some flatworms that were conditioned
to feeding on barnacles always preferred barnacles and would
not eat oysters, whereas worms conditioned to oysters fed
on both oysters and barnacles. Because barnacles begin set-
ting in April or May in the Chesapeake, some 2 to 3 months
before oysters, flatworms could become conditioned to
feeding on barnacles and thus not attack spat. Although
oysters do not appear to be the main prey preference of S.
ellipticus (Christensen, 1973), there could be circumstances
during and after the oyster setting season when young flat-
worms become conditioned to newly set spat and feed heavily
on them.

The blue crab Callinectes sapidus Rathbun is extreme-
ly abundant in Chesapeake Bay and can grow to more than
20 cm in carapace width (Pearson, 1948; Van Engel, 1958;
Abbe, 1983). St can tolerate a salinity range from fresh water
to hypersaline conditions and is found throughout the
Chesapeake wherever oysters are found (Van Engel, 1958;
Lippson and Lippson, 1984; Williams, 1984). The blue crab
is not regarded as a serious pest of oyster grounds in open
waters (Van Engel, 1958), but it can be destructive of
transplants of young sets when other food is scarce. Because
spat generally set on shells or live oysters (or other solid ob-
jects) they are difficult for crabs to handle, but small single
oysters and thin-she!led cultchiess spat and seed are easily
manipulated and eaten; Krantz and Chamberlin (1 978) noted
destruction of 79 to 99% of 3- to 40- mm cultchiess oysters
in a month from crab predation. Although oysters are not their

ABBE: FACTORS LIMITING OYSTER RECRUITMENT

65

SPAT SUBLEGAL LEGAL

Fig. 3. Ratios of inside to outside densities for three size classes of oysters at three depth ranges on the intake curtain wall at the Calvert
Cliffs Nuclear Power Plant during 1980.

preferred food, blue crabs can decimate newly settled or
seeded grounds because of their large size, voracious ap-
petites, and abundance.

Mud crabs (Xanthidae) also feed on spat and small
oysters. Eurypanopeus depressus (Smith) and Panopeus herb-
stii H. Milne Edwards both were shown by McDermott (1960)
to prey on oysters in New Jersey. The latter was the most
destructive of the five species of mud crabs studied, feeding
on 1- and 2-year-old oysters at a rate of 0.15 per crab per
day. Both of these crabs are widely distributed throughout
Chesapeake Bay (Lippson and Lippson, 1984) as is
Rhithropanopeus harrisii (Gould) that can also prey on spat
(Krantz and Chamberlin, 1978). P. herbstii is the most destruc-
tive probably because it is larger (up to 62 mm in carapace
width [Williams, 1984]) than the others. E. depressus is the
next largest of the three, but grows to only about 25 mm. Den-
sity of mud crabs, however, can be more important to total
predation than individual size. Elner and Lavoie (1983)
showed that Neopanope sayi (Smith) (carapace width 14 to
23 mm) fed on 2- to 9-mm oysters at a rate of 0.44 per crab

per day in New Brunswick, Canada. Mud crabs in Long Island
Sound feed on attached spat up to 10 mm in length and unat-
tached spat up to 25 mm. They can account for 12% of the
total spat mortality and up to 50% within certain beds
(MacKenzie, 1970).

Krantz and Chamberlin (1978) listed several fish
species as probable predators of oyster spat in the Maryland
Chesapeake Bay. These included oyster toadfish Opsanus
tau (Linnaeus), croaker Micropogon undulatus (Linnaeus),
spot Leiostomus xanthurus Lacepede, and the cownose ray
Rhinoptera bonasus (Mitchill). The cownose ray is probably
the most destructive of these; oysters and razor clams are
its major food items (Hildebrand and Schroeder, 1928).
Schwartz (1964) stated that cownose rays can feed in
devastating proportions on oysters and hard clams. Rays col-
lected while feeding over shallow sand or mud flats in the
lower York River, Virginia consumed primarily soft shell
clams, Mya arenaria Linnaeus (Smith and Merriner, 1985),
but these authors also observed serious damage to commer-
cial oyster and clam beds caused by feeding schools of

66

LARVAL ENTRAINMENT

cownose rays.

In the lower Bay the black drum Pogonias cromis (Lin-
naeus) is a potential predator of large and small oysters. It
is common from New York southward and abundant from the
Carolinas to the Rio Grande (Bigelow and Schroeder, 1953),
but it seldom penetrates far into the Maryland portion of the
Chesapeake (Lippson and Lippson, 1984). Adult size ranges
from 9 to 18 kg, but can reach up to 66 kg (Bigelow and
Schroeder, 1953). The fish possesses strong pharyngeal teeth
used to crush oysters and other shellfish on which it feeds
(Galtsoff , 1 964; Cave and Cake, 1 980). The degree to which
rays and drum feed on spat set on cultch in the Bay is
unknown, but spat set on small shell fragments and cultchless
spat are both subject to predation.

A group of predators not found in the Bay when oysters
are setting, but that arrives during autumn and overwinters
includes several species of diving ducks. About 65% of the
diet of the black (common) scoter Melanitta nigra (Linnaeus)
consists of molluscs (6% oysters) (Kortright, 1967). The diet
of the white-winged scoter M. fusca (Linnaeus) is about 75%
molluscs (14% oysters). The diet of white-winged scoters liv-
ing near commercial oyster beds in Washington, however,
consisted of 50 to 70% oysters (Galtsoff, 1964; Kortright,
1967). Another study cited by Galtsoff (1964) indicated that
up to 80% of the diets of greater and lesser scaup, Aythya
marila (Linnaeus) and A. affinis (Eyton), respectively, con-
sisted of oysters. Extent of damage to oyster populations in
Chesapeake Bay caused by these birds is also unknown, but
since they number in the tens of thousands (Lippson, 1973)
their potential as predators is large.

Competition for space is intense between oyster
pediveligers and free swimming larvae of other sessile
species such as barnacles, encrusting bryozoans, mud-tube-
building worms and amphipods, colonial hydroids, and
tunicates (Cory, 1967; Kennedy, 1980; Abbe and Yates,
1982). This competition continues after setting is completed
and spat can be killed by barnacles, encrusting bryozoans,
or other spat growing over the upper valve preventing open-
ing or between the two valves preventing closing (MacKenzie,
1970). Mud tubes of various polychaete worms and am-
phipods Corophium spp. as well as tunicates can also cover
and suffocate small spat. Galtsoff (1964) described heavy in-
festations of the tunicate Molgula manhattensis (DeKay) in
the Oyster River, Massachusetts and in the Chester River,
Maryland where dredged oysters could not be seen for the
tunicates. Cory (1967) observed massive colonies of M.
manhattensis in the Patuxent River, Maryland, with peak at-
tachment in July and August. Abbe (1978) observed such
heavy growth of M. manhattensis among tray-held oysters
near Calvert Cliffs in Chesapeake Bay during autumn that
even 85-mm oysters were unable to grow; spat would almost
surely be killed by such overgrowth.

Diseases can devastate adult oyster populations as
evidenced by what is now commonly known as “MSX”
caused by Haplosporidium nelsoni (Haskin, Stauber, and
Mackin), a related disease caused by Haplosporidium costalis
Wood and Andrews or SSO (for Seaside organism), and
“Dermo” caused by Perkinsus marinus (Mackin, Owen, and

Collier) (Wood and Andrews, 1962; Galtsoff, 1964; Andrews,
1966, 1967, 1968, 1982; Lippson, 1973; Cake, 1983a). New-
ly set spat, however, and young oysters less than 2 years old
seem relatively unaffected by these diseases (Andrews and
Hewatf, 1957; Andrews, 1964; Cake, 1983a). There are
diseases among larvae and spat caused by bacterial
pathogens (Guillard, 1959; Tubiash etal., 1965; Sinderman,
1970; Elston and Leibovitz, 1980), but most studies have
referenced conditions found in cultures and hatcheries; in-
formation on field mortalities of larvae and spat caused by
pathogenic bacteria or viruses is lacking (Kennedy and
Breisch, 1981).

Productive oyster bottom is often adjacent to barren
sand bottom (Haven and Whitcomb, 1983; Abbe, 1984) and
sand can be another cause of mortality among young oysters.
Movement of sand and its abrasive quality can destroy young
oysters (Kennedy and Breisch, 1981), and sand moved by
storms can bury and suffocate spat. Dunnington (1968)
demonstrated by experimental burial of oysters that survival
time was as short as 2 days in summer. In October 1982 a
severe storm in the upper Chesapeake transported enough
sand to completely bury crab pots 0.5 m tall (personal obser-
vation). Areas 1 and 3 of Flag Pond Bar near Calvert Cliffs
(Fig. 2), which were of only marginal productivity before this
storm, were little more than barren sand bottom when
examined in early 1983 (Abbe, 1984). This volume of sand
would have easily buried and killed any recently set spat.

Spat can also be buried by deposition of fine sediments
and by biodeposition (deposits of feces and pseudofeces of
oysters and other organisms). The Chesapeake Bay, like
other estuaries, is rapidly filling with sediments from rivers,
shore erosion, organisms inhabiting it, and the ocean
(Galtsoff, 1964; Schubel, 1977; Cake, 1983a). Some of this
filling results from natural weathering of rocks and soils and
cannot be completely halted; e.g., natural erosion along the
western shore of Chesapeake Bay between 1847 and 1942
caused the shoreline to recede up to 140 m (Schultz and
Ashby, 1967). Other filling results from increased sedimen-
tation due to unsound agricultural practices and urbaniza-
tion (Roberts and Pierce, 1376). Haven etal. (1981) indicated
increased sedimentation in the James River during the last
three decades and suggested that it could have affected spat
setting. Where tidal currents are strong enough to carry away
sediments and biodeposits, oysters are generally productive,
but where currents are slow and sediments accumulate faster
than they can be removed, burial can occur. Addition of shell
cultch to areas where sedimentation rates are high, in an ef-
fort to increase production, may be wasted effort as shells
can act as baffles that slow the current and increase the
sedimentation rate (Galtsoff, 1964).

Under certain circumstances biodeposition can pose
an even greater threat to oyster populations than natural
sedimentation. Haven and Morales-Alamo (1966) determined
that oysters deposited filtered material about seven times
faster than it settled by gravity. Lund (1957) calculated that
if oysters covered a hectare of bottom, they would deposit
18.7 metric tons (dry weight) of fecal material in 1 1 days. Abbe
(1985) observed the effects of sedimentation and biodeposi-

ABBE: FACTORS LIMITING OYSTER RECRUITMENT

67

tion on a bed of shells up to 30 cm or more thick planted by
the state of Maryland near Calvert Cliffs in 1982. Spat set
throughout this entire shell mass in 1982, and in 1983 the
densities of 30- to 60-mm oysters were as high as 400 to 800
per m2. By 1984 these densities were reduced to 50 to 60
oysters per m2, all confined to the upper cultch layer; oysters
below these had been smothered by sediments and
biodeposits.

SUMMARY

It is apparent that many factors can work against suc-
cessful recruitment of oysters. These vary in severity at dif-
ferent stages of embryonic, larval, and postmetamorphic
development. Four conditions are crucial to successful recruit-
ment. One is the presence of an abundant population of
spawning adults. Inadequate broodstocks cannot produce
enough larvae to maintain or expand populations. Small
broodstock size has been suggested as one reason for the
decline in productivity of the seed beds in the James River,
Virginia since 1 960 when MSX killed most adults in the River
downstream from the seed beds (Andrews, 1983; Haven and
Fritz, 1985). A second condition is clean water for pollutants
can prevent eggs and embryos from reaching even the early
larval stage. A third requirement is the retention of adequate
numbers of larvae in the estuary as a result of its shape (trap
type) or by water circulation patterns and larval movements.
Clearly, without larvae that have survived to the pediveliger
stage no setting can occur. The fourth factor is the availability
of clean hard substratum (shell or similar material); without
a place to set, pediveligers will eventually die. This condi-
tion is under more immediate human control than the others,
as shell material can be dredged (fossil shells) or collected
(fresh shells) and planted on grounds that are thinly populated
or barren, but firm enough to support it. There are many areas
within Chesapeake Bay ihat could be more productive simp-
ly by the addition of shell. For the oyster fishery to begin a
recovery, however, all four conditions must operate together.
Otherwise annual production may never reach the four to five
million bushel potential.

LITERATURE CITED

Abbe, G. R. 1978. Oyster tray studies. In: Non-radiological environ-
mental monitoring report, Calvert Cliffs Nuclear Power Plant,
January-December 1977, pp. 10.1-1 through 10.1-24. The
Academy of Natural Sciences, Philadelphia.

Abbe, G. R. 1981. A study of the curtain-wall oyster population at
the Calvert Cliffs Nuclear Power Plant during 1980. Report
Number 81-26. The Academy of Natural Sciences,
Philadelphia. 29 pp.

Abbe, G. R. 1983. Blue crab ( Callinectes sapidus Rathbun) popula-
tions in mid-Chesapeake Bay in the vicinity of the Calvert Cliffs
Nuclear Power Plant, 1968-81 . Journal of Shellfish Research
3:183-193.

Abbe, G. R. 1984. Oyster population density on Flag Pond Bar near
the Calvert Cliffs Nuclear Power Plant in central Chesapeake
Bay, 1983. Report Number 84-3. The Academy of Natural
Sciences, Philadelphia. 38 pp.

Abbe, G. R. 1985. Oyster population density surveys in central
Chesapeake Bay near the Calvert Cliffs Nuclear Power Plant,
1984. Report Number 85-4. The Academy of Natural Sciences,
Philadelphia. 37 pp.

Abbe, G. R. and W. L. Yates, Jr. 1982. Submerged substrate studies
on the Chesapeake Bay at Calvert Cliffs, Maryland, January
- December 1979 and 1980 for the Baltimore Gas and Elec-
tric Company. Report Number 82-9. The Academy of Natural
Sciences, Philadelphia. 47 pp.

Andrews, J. D. 1956. Trapping oyster drills in Virginia. Part I. The
effects of migration and other factors on catch. Proceedings
of the National Shellfisheries Association 46:140-154.

Andrews, J. D. 1964. Oyster mortality studies in Virginia. IV. MSX
in James River public seed beds. Proceedings of the National
Shellfisheries Association 53:65-84.

Andrews, J. D. 1966. Oyster mortality studies in Virginia. V. Epi-
zootiology of MSX, a protistan pathogen of oysters. Ecology
47:19-31.

Andrews, J. D. 1967. Interaction of two diseases of oysters in natural
waters. Proceedings of the National Shellfisheries Association
57:38-49.

Andrews, J. D. 1968. Oyster mortality studies in Virginia. VII. Review
of epizootiology and origin of Minchinia nelsoni. Proceedings
of the National Shellfisheries Association 58:23-36.

Andrews, J. D. 1982. Epizootiology of late summer and fall infec-
tions of oysters by Haplosporidium nelsoni, and comparison
to annual life cycle of Haplosporidium costalis, a typical
haplosporidium. Journal of Shellfish Research 2:15-23.

Andrews, J. D. 1983. Transport of bivalve larvae in James River,
Virginia. Journal of Shellfish Research 3:29-40.

Andrews, J. D. and W. G. Hewatt. 1957. Oyster mortality studies in
Virginia. II. The fungus disease caused by Dermocystidium
marinum in oysters of Chesapeake Bay. Ecological
Monographs 27:1-26.

Bigelow, H. B. and W. C. Schroeder. 1953. Fishes of the Gulf of
Maine. United States Fish and Wildlife Service, Fishery Bulletin
74:1-577.

Bishop, J. W. 1967. Feeding rates of the ctenophore Mnemiopsis
leidyi. Chesapeake Science 8:259-261.

Butler, P. A. 1949. Gametogenesis in the oyster under conditions of
depressed salinity. Biological Bulletin 96:263-269.

Cake, E. W., Jr. 1983a. Habitat suitability index models: Gulf of Mex-
ico American oyster. United States Department of Interior, Fish
and Wildlife Service, FWS/OBS-82/10.57. 37 pp.

Cake, E. W., Jr. 1983b. Symbiotic associations involving the southern
oyster drill Thais haemastoma floridana (Conrad) and
macrocrustacens in Mississippi waters. Journal of Shellfish
Research 3:1 17-128.

Calabrese, A. 1972. How some pollutants affect embryos and lar-
vae of American oyster and hard-shell clam. Marine Fisheries
Review 34:66-77.

Calabrese, A., R. S. Collier, D. A. Nelson, and J. R. Maclnnes. 1973.
The toxicity of heavy metals to embryos of the American
oyster, Crassostrea virginica. Marine Biology 18:162-166.

Calabrese, A. and H. C. Davis. 1966. The pH tolerance of embryos
and larvae of Mercenaria mercenaria and Crassostrea virginica.
Biological Bulletin 131:427-436.

Calabrese, A. and H. C. Davis. 1967. Effects of ‘‘soft’’ detergents
on embryos and larvae of the American oyster ( Crassostrea
virginica). Proceedings of the National Shellfisheries Associa-
tion 57:11-16.

Calabrese, A., J. R. Maclnnes, D. A. Nelson, and J. E. Miller. 1977.
Survival and growth of bivalve larvae under heavy-metal stress.
Marine Biology 41:179-184.

68

LARVAL ENTRAINMENT

Carriker, M. R. 1951. Ecological observations on the distribution of
oyster larvae in New Jersey estuaries. Ecological Monographs
21:19-38.

Carriker, M. R. 1955. Critical review of biology and control of oyster
drills Urosalpinx and Eupleura. United States Fish and Wildlife
Service, Special Scientific Report, Fisheries Service
148:1-150.

Carriker, M. R. 1961. Comparative functional morphology of boring
mechanisms in gastropods. American Zoologist 1:263-266.

Carriker, M. R. 1981. Shell penetration and feeding by naticacean
and muricacean predatory gastropods: a synthesis.
Malacologia 20:403-422.

Cave, R. N. and E. W. Cake, Jr. 1980. Observations on the preda-
tion of oysters by the black drum Pogonias cromis (Linnaeus)
(Sciaenidae). Proceedings of the National Shellfisheries
Association 70:121.

Christensen, D. J. 1973. Prey preference of Stylochus ellipticus in
Chesapeake Bay. Proceedings of the National Shellfisheries
Association 63:35-38.

Cory, R. L. 1967. Epifauna of the Patuxent River Estuary, Maryland,
for 1963 and 1964. Chesapeake Science 8:71-89.

Crisp, D. J. 1967. Chemical factors inducing settlement in Crassostrea
virginica (Gmelin). Journal of Animal Ecology 36:329-335.

Cunningham, P. A. 1979. The use of bivalve molluscs in heavy metal
pollution research. In: Marine Pollution: Functional Responses,
W. B. Vernberg, A. Calabrese, F. P. Thurberg, and F. J. Vern-
berg, eds., pp. 183-221. Academic Press, New York.

Davis, H. C. 1953. On food and feeding of larvae of the American
oyster, C. virginica. Biological Bulletin 104:334-350.

Davis, H. C. 1958. Survival and growth of clam and oyster larvae
at different salinities. Biological Bulletin 114:296-307.

Davis, H. C. 1961. Effects of some pesticides on eggs and larvae
of oysters ( Crassostrea virginica) and clams ( Venus
mercenaria). Commercial Fisheries Review 23(12):8-23.

Davis, H. C. and A. D. Ansell. 1962. Survival and growth of larvae
of the European oyster, O. edulis, at lowered salinities.
Biological Bulletin 122:33-39.

Davis, H. C. and A. Calabrese. 1964. Combined effects of
temperature and salinity on development of eggs and growth
of larvae of M. mercenaria and C. virginica. Biological Bulletin
63:643-655.

Davis, H. C. and H. Hidu. 1969a. Effects of turbidity-producing
substances in sea water on eggs and larvae of three genera
of bivalve mollusks. Veliger 11:316-323.

Davis, H. C. and H. Hidu. 1969 b. Effects of pesticides on embryonic
development of clams and oysters and on survival and growth
of the larvae. United States Fish and Wildlife Service, Fishery
Bulletin 67:393-404.

Dunnington, E. A. 1968. Survival time of oysters after burial at various
temperatures. Proceedings of the National Shellfisheries
Association 58:101-103.

Elner, R. W. and R. E. Lvoie. 1983. Predation on American oysters
(Crassostrea virginica [Gmelin]) by American lobsters
(Homarus americanus Milne-Edwards), rock crabs (Cancer ir-
roratus Say), and mud crabs (Neopanope sayi [Smith]). Jour-
nal of Shellfish Research 3:129-134.

Elston, R. and L. Leibovitz. 1980. Pathogenesis of experimental
vibriosis in larval American oysters, Crassostrea virginica.
Canadian Journal of Fisheries and Aquatic Sciences
37:964-978.

Engle, J. B. 1947. Commercial aspects of the upper Chesapeake
Bay oyster bars in the light of recent oyster mortalities. Pro-
ceedings of the National Shellfisheries Association (1 946):42-46.

Fritz, L. W., R. A. Lutz, M. A. Foote, C. L. Van Dover, and J. W. Ewart.

1984. Selective feeding and grazing rates of oyster
(Crassostrea virginica) larvae on natural phytoplankton
assemblages. Estuaries 7:513-518.

Galtsoff, P. S. 1964. The American oyster, Crassostrea virginica
Gmelin. United States Fish and Wildlife Service, Fishery
Bulletin 64:1-480.

Guillard, R. R. 1958. Some factors in the use of nannoplankton
cultures as food for larval and juvenile bivalves. Proceedings
of the National Shellfisheries Association 48:134-142.

Guillard, R. R. 1959. Further evidence of the destruction of bivalve
larvae by bacteria. Biological Bulletin 117:258-263.

Hadfield, M. G. 1984. Settlement requirements of molluscan larvae:
new data on chemical and genetic roles Aquaculture
39:283-298.

Haven, D. S. and L. W. Fritz. 1985. Setting of the American oyster
Crassostrea virginica in the James River, Virginia, USA: tem-
poral and spatial distribution. Marine Biology 86:271-282.

Haven, D. S., W. J. Hargis, Jr., J. G. Loesch, and J. P. Whitcomb.
1977. The effect of tropical storm Agnes on oysters, hard
clams, soft clams, and oyster drills in Virginia. In: The effects of
tropical storm Agnes on the Chesapeake Bay estuarine system,
A. M. Anderson, ed., pp. 488-507. Chesapeake Research
Consortium Publication Number 54. The Johns Hopkins Uni-
versity Press. Baltimore.

Haven, D. S., W. J. Hargis, Jr. and P. C. Kendall. 1981 . The oyster
industry of Virginia: its status, problems and promise. A com-
prehensive study of the oyster industry in Virginia, 2nd edi-
tion. Special Papers in Marine Science Number 4, Virginia In-
stitute of Marine Science, Gloucester Point. 1024 pp.

Haven, D. S. and R. Morales-Alamo. 1966. Aspects of biodeposition
by oysters and other invertebrate filter feeders. Limnology and
Oceanography 11:487-498.

Haven, D. S. and J. P. Whitcomb. 1983. The origin and extent of
oyster reefs in the James River, Virginia. Journal of Shellfish
Research 3:141-151.

Hidu, H., W. H. Roosenburg, K. G. Drobeck, A. J. McErlean, and
J. A. Mihursky. 1974. Thermal tolerance of oyster larvae,
Crassostrea virginica Gmelin, as related to power plant opera-
tion. Proceedings of the National Shellfisheries Association
64:102-110.

Hildebrand, S. F. and W. C. Schroeder. 1928. Fishes of Chesa-
peake Bay. Bulletin of the United States Bureau of Fisheries
43:1-388.

Hyman, L. H. 1940. The polyclad flatworms of the Atlantic coast of
the United States and Canada. Proceedings of the United
States National Museum 89:449-495.

Kennedy, V. S. 1980. Comparison of recent and past patterns of
oyster settlement and seasonal fouling in Broad Creek and
Tred Avon River, Maryland. Proceedings of the National
Shellfisheries Association 70:36-40.

Kennedy, V. S.and L. L. Breisch. 1981. Maryland’s oysters: research
and management. Maryland Sea Grant Number UM-SG-
TS-81-04, College Park. 286 pp.

Kennedy, V. S. and L. B. Krantz. 1982. Comparative gametogenic
and spawning patterns of the oyster Crassostrea virginica
(Gmelin) in central Chesapeake Bay. Journal of Shellfish
Research 2:133-140.

Kortright, F. H. 1967. The Ducks, Geese, and Swans of North
America. Stackpole Company. Harrisburg, Pennsylvania and
Wildlife Management Institute, Washington, District of Colum-
bia. 476 pp.

Krantz, G. E. 1982. Summary and hatchery history. In: Oyster hatch-
ery technology series, J. Greer, ed., pp. 1-40. Maryland Sea
Grant, Marine Advisory Publication Number UM-SG-

ABBE: FACTORS LIMITING OYSTER RECRUITMENT

69

MAP-82-01, College Park.

Krantz, G. E. and J. V. Chamberlin. 1978. Blue crab predation on
cultchless oyster spat. Proceedings of the National
Shellfisheries Association 68:38-41.

Krantz, G. E., H. A. Davis and D. W. Webster. 1982. Maryland oyster
spat survey, tall 1981 . Maryland Sea Grant Technical Report
UM-SG-TS-82-02, College Park. 14 pp.

Krantz, G. E. and H. A. Davis. 1983. Maryland oyster spat survey,
fall 1982. Maryland Sea Grant Technical Report UM-SG-
TS-83-01 , College Park. 14 pp.

Kunkle, D. E. 1957. The vertical distribution of oyster larvae in
Delaware Bay. Proceedings of the National Shellfisheries
Association 48:90-91.

Landers, W. S. and R. W. Rhodes, Jr. 1970. Some factors influenc-
ing predation by the flatworm, Stylochus ellipticus (Girard), on
oysters. Chesapeake Science 11:55-60.

Lippson, A. J. 1973. The Chesapeake Bay in Maryland: An Atlas of
Natural Resources. The Johns Hopkins University Press,
Baltimore. 55 pp.

Lippson, A. J. and R. L. Lippson. 1984. Life in the Chesapeake Bay.
The Johns Hopkins University Press, Baltimore. 229 pp.

Loosanoff, V. L. 1953. Behavior of oysters in water of low salinities.
Proceedings of the National Shellfisheries Association
(1952): 135-1 51.

Loosanoff, V. L. 1956. Two obscure oyster enemies in New England
waters. Science 123:1119-1120.

Loosanoff, V. L. 1 969. Maturation of gonads of oysters, Crassostrea
virginica, of different geographical areas subjected to relatively
low temperatures. Veliger 11:153-163.

Losee, E. 1979. Relationship between larval and spat growth rates
in the oyster ( Crassostrea virginica). Aquaculture 16:123-126.

Lund, E. J. 1957. A quantitative study of clearance of a turbid medium
and feeding by the oyster. Publications of the Institute of Marine
Science, University of Texas 4:296-312.

Lunz, J. D. 1985. An analysis of available information concerning
the entrainment of oyster larvae during hydraulic cutterhead
dredging operations with commentary on the reasonableness
of seasonally restrictive dredging windows. Prepared for U.S.
Army Engineer District, Baltimore by Environmental Resources
Division, Waterways Experiment Station, Vicksburg, Mississip-
pi. 9 pp.

MacKenzie, C. L., Jr. 1970. Causes of oyster spat mortality, condi-
tions of oyster setting beds, and recommendations for oyster
bed management. Proceedings of the National Shellfisheries
Association 60:59-67.

MacKenzie, C. L., Jr. 1977. Sea anemone predation on larval oysters
in Chesapeake Bay (Maryland). Proceedings of the National
Shellfisheries Association 67:113-117.

MacKenzie, C. L., Jr. 1981. Biotic potential and environmental
resistance in the American oyster ( Crassostrea virginica) in
Long Island Sound. Aquaculture 22:229-268.

MacKenzie, C. L., Jr. 1983. To increase oyster production in the north-
eastern United States. Marine Fisheries Review 45(3): 1-22.

Mackie, G. 1969. Quantitative studies of feeding in the oyster,
Crassostrea virginica. Proceedings of the National Shellfisheries
Association 59:6-7.

Mackiernan, G. B., D. A. Flemer, W. Nehlsen, and V. K. Tippie. 1983.
State of the Bay. In: Chesapeake Bay: a framework for action,
pp. 15-35. United States Environmental Protection Agency,
Philadelphia.

Mansueti, R. 1955. The sea nettle, Chesapeake Bay’s troublesome
summer jellyfish. Maryland Tidewater News 12 (3, Supplement
7): 1-2.

May, E. B. 1971. A survey of the oyster and oyster shell resources

of Alabama. Alabama Marine Resources Bulletin Number 4.
Dauphin Island, Alabama. 53 pp.

May, E. B. 1972. The effect of floodwater on oysters in Mobile Bay.
Proceedings of the National Shellfisheries Association 62:67-71 .

McDermott, J. 1960. The predation of oysters and barnacles by crabs
of the family Xanthidae. Proceedings of the Pennsylvania
Academy of Science 34:1 99-21 1 .

Medcof, J. C. 1939. Larval life of the oyster ( Ostrea virginica) in
Bidefore River. Journal of the Fisheries Research Board of
Canada 4:287-301 .

Medcof, J. C. 1955. Day and night characteristics of spatfall and the
behavior of oyster larvae. Journal of the Fisheries Research
Board of Canada 12:270-286.

Meritt, D. W. 1 977. Oyster spat set on natural cultch in the Maryland
portion of the Chesapeake Bay (1939-1975). University of
Maryland, Center for Environmental and Estuarine Studies,
Special Report Number 7. Cambridge. Ill pp.

Mountford, K. 1980. Occurrence and predation by Mnemiopsis leidyi
in Barnegat Bay, New Jersey. Estuarine and Coastal Marine
Science 10:393-402.

Nelson, T. C. and A. Chestnut. 1945. Some observations on the
transplantation of two weeks old set and of older oysters. Pro-
ceedings of the National Shellfisheries Association (1 944):3 pp.

Newkirk, G. F. 1 981 . Do fast growing oyster larvae produce fast grow-
ing adult oysters? Journal of Shellfish Research 1:120.

Officer, C. B., R. B. Biggs, J. L. Taft, L. E. Cronin, M. A. Tyler, and
W. R. Boynton. 1984. Chesapeake Bay anoxia: origin, develop-
ment, and significance. Science 223:22-27.

Pearse, A. S. and G. W. Wharton. 1983. The oyster “leech”,
Stylochus inimicus Palombi, associated with oysters on the
coast of Florida. Ecological Monographs 8:605-655.

Pearson, J. C. 1948. Fluctuations in the abundance of the blue crab
in Chesapeake Bay. United States Fish and Wildlife Service
Research Report 14:26 pp.

Price, K. S., Jr. and D. Maurer. 1971. Holding and spawning Delaware
Bay oysters ( Crassostrea virginica) out of season. II.
Temperature requirements for maturation of gonads. Pro-
ceedings of the National Shellfisheries Association 61 :29-34.

Pritchard, D. W. 1953. Distribution of oyster larvae in relation to
hydrographic conditions. Proceedings of the Gulf and Carib-
bean Fisheries Institute 5:123-132.

Provenzano, A. J., Jr. 1961 . Effects of the flatworm Stylochus ellip-
ticus (Girard) on oyster spat in two salt water ponds in
Massachusetts. Proceedings of the National Shellfisheries
Association 50:83-88.

Renzoni, A. 1975. Toxicity of three oils to bivalve gametes and lar-
vae. Marine Pollution Bulletin 6:125-128.

Richardson, L. B., D. T. Burton, and A. M. Stavola. 1982. A com-
parison of ozone and chlorine toxicity to three life stages of
the American oyster Crassostrea virginica. Marine Environmen-
tal Research 6:99-113.

Ritchie, T. P. and R. W. Menzel. 1969. Influence of light on larval
settlement of American oysters. Proceedings of the National
Shellfisheries Association 59:1 16-120.

Roberts, W. P. and J. W. Pierce. 1976. Deposition in upper Patux-
ent Estuary, Maryland, 1968-1969. Estuarine and Coastal
Marine Science 4:267-280.

Roosenburg, W. H., J. C. Rhoderick, R. M. Block, V. S. Kennedy,
S. R. Gullans, S. M. Vreenegoor, A. Rosenkranktz, and C.
Collette. 1980. Effects of chlorine-produced oxidants on sur-
vival of larvae of the oyster Crassostrea virginica. Marine
Ecological Progress Series 3:93-96.

Ruddy, G. M., S. Y. Feng, and G. S. Campbell. 1975. The effect of
prolonged exposure to elevated temperatures on the

70

LARVAL ENTRAINMENT

biochemical constituents, gonadal development and shell
deposition of the American oyster, Crassostrea virginica. Com-
parative Biochemistry and Physiology 5113:157-164.

Schubel, J. R. 1977. Effects of Agnes on the suspended sediment
of the Chesapeake Bay and contiguous shelf waters. In: The
effects of tropical storm Agnes on the Chesapeake Bay
estuarine system, J. R. Schubel, ed., pp. 179-200. Chesapeake
Research Consortium Publication Number 54. The Johns
Hopkins University Press, Baltimore.

Schultz, L. P. and W. Ashby. 1 967. An analysis of an attempt to con-
trol beach erosion in Chesapeake Bay, at Scientists Cliffs,
Calvert County, Maryland. Chesapeake Science 8:237-252.

Shaw, R., D. C. Arnold, and W. B. Stallworthy. 1970. Effects of light
on spat settlement of the American oyster (Crassostrea
virginica). Journal of the Fisheries Research Board of Canada
27:743-748.

Sindermann, C. J. 1970. Principal Diseases of Marine Fish and
Shellfish. Academic Press, New York. 369 pp.

Smith, J. W. and J. V. Merriner. 1985. Food habits and feeding
behavior of the cownose ray, Rhinoptera bonasus, in lower
Chesapeake Bay. Estuaries 8:305-310.

Sparks, A. K., J. L. Boswell, and J. G. Mackin. 1958. Studies on the
comparative utilization of oxygen by living and dead oysters.
Proceedings of the National Shellfisheries Association
48:92-102.

Schwartz, F. J. 1964. Fishes of Isle of Wight and Assawoman Bays
near Ocean City, Maryland. Chesapeake Science 5:172-193.

Taft, J. L., E. O. Hartwig, and R. Loftus. 1980. Seasonal oxygen deple-
tion in Chesapeake Bay. Estuaries 3:242-247.

Tubiash, H. S., P. E. Chanley, and E. Leifson. 1965. Bacillary
necrosis, a disease of larval and juvenile bivalve molluscs.
I. Etiology and epizootiology. Journal of Bacteriology

90:1036-1044.

Ukeles, R. 1971. Nutritional requirements in shellfish culture. In: Ar-
tificial propogation of commercially valuable shellfish, K. S.
Price, Jr. and D. L. Maurer, eds., pp. 43-66. University of
Delaware, Newark.

Ulanowicz, R. E., W. C. Caplins and E. A. Dunnington. 1980. The fore-
casting of oyster harvest in central Chesapeake Bay. Estuarine
and Coastal Marine Science 11:101-106.

Van Engel, W. A. 1958. The blue crab and its fishery in Chesapeake
Bay. I. Reproduction, early development, growth, and migra-
tion. Commercial Fisheries Review 20(6):6-17.

Webster, J. R. and R. Z. Medford. 1961. Flatworm distribution and
associated oyster mortality in Chesapeake Bay. Proceedings
of the National Shellfisheries Association 50:89-95.

Williams, A. B. 1984. Shrimps, Lobsters, and Crabs of the Atlantic
Coast of the Eastern United States, Maine to Florida. Smith-
sonian Institution Press, Washington. 550 pp.

Wolfe, D. A. 1977. Fate and Effects of Petroleum Hydrocarbons in
Marine Ecosystems and Organisms. Pergamon Press, New
York. 478 pp.

Wood, J. L. and J. D. Andrews. 1962. Haplosporidium costale
(Sporozon) associated with a disease of Virginia oysters.
Science 136:710-711.

Wood, L. and W. J. Hargis, Jr. 1971. Transport of bivalve larvae in
a tidal estuary. In: Fourth European Marine Biological Sym-
posium, D. J. Crisp, ed., pp. 29-44. Cambridge University
Press, Cambridge.

Zaroogian, G. E. and G. Morrison. 1981. Effect of cadmium body
burdens in adult Crassostrea virginica on fecundity and viability
of larvae. Bulletin of Environmental Contamination and Tox-
icology 27:344-348.

ENTRAINMENT OF OYSTER LARVAE BY HYDRAULIC CUTTERHEAD

DREDGING OPERATIONS:

WORKSHOP CONCLUSIONS AND RECOMMENDATIONS

MELBOURNE R. CARRIKER1, MARK W. LASALLE2, ROGER MANN3

and

DONALD W. PRITCHARD4,5

^COLLEGE OF MARINE STUDIES, UNIVERSITY OF DELAWARE,

LEWES, DELAWARE 19958, U.S.A.

ENVIRONMENTAL LABORATORY, U.S. ARMY ENGINEER WATERWAYS
EXPERIMENT STATION, P.O. BOX 631, VICKSBURG, MISSISSIPPI 39180, U.S.A.

VIRGINIA INSTITUTE OF MARINE SCIENCES, COLLEGE OF WILLIAM AND MARY,
GLOUCESTER POINT, VIRGINIA 23062, U.S.A.

4MARINE SCIENCES RESEARCH CENTER, STATE UNIVERSITY OF NEW YORK
AT STONY BROOK, STONY BROOK, NEW YORK 11794, U.S.A.

ABSTRACT

Conclusions and recommendations developed during this workshop concerning entrainment of
oyster larvae by hydraulic cutterhead dredging operations are presented, including: a) suggestions
for cooperative interactions between opposing state and federal agencies over the concerns raised
about dredge-induced oyster larval mortality, and b) a numerical model of oyster larval entrainment
based on a set of conservative assumptions about oyster larval distribution and dimensions of bodies
of water. Recognition of variability of estuarine systems in time and space by all parties involved could
provide flexibility in making decisions about dredging timetables. Application of the proposal model
of larval entrainment under “worst case scenarios’’ produced estimates of dredge-induced larval mor-
talities between 0.005 and 0.3%, differing substantially from rates proposed by Carter (1986) on the
order of 12.6 to 55.4%. Additional consequences of dredging, having possible indirect effects on oyster
larvae, include: resuspended sediment, siltation and dissolved oxygen demand. We concluded that
our understanding of both direct and indirect effects of dredge-induced impacts on oyster larvae are
minimal.

Entrainment of oyster larvae by hydraulic cutterhead
dredging operations has become the subject of a dilemma
between one agency charged with protecting a valuable
natural resource (Maryland Department of Natural Resources)
and a second charged with maintaining navigable waterways
(U.S. Army Corps of Engineers). This workshop brought
together recognized experts to review and discuss what is
known about oyster biology and hydraulic dredging and to
attempt to place the subject of entrainment in perspective.
After the reviews and discussions, workshop participants were
asked to: a) determine if a practical numerical model of lar-
val entrainment could be constructed, and if so, define the

5Authorship is alphabetical

compartments and relationships between compartments and
produce a set of computations for approximating compart-
ment values; b) determine the feasibility of field verification
of proposed model(s) and; (c) propose possible methods of
monitoring a dredging project routinely for the purpose of
determining the need to restrict or alter dredging opera-
tions.

Responses to these questions come from two
workshop subsections, both working independently but ad-
dressing the same questions. Objectives of this paper are
a) to report the conclusions and recommendations of the
workshop subsections, and b) to attempt to placed dredge-
induced oyster larval mortality in perspective relative to

American Malacological Bulletin, Special Edition No. 3(1 986):71 -74

71

72

LARVAL ENTRAINMENT

oyster productivity.

CONCLUSIONS AND RECOMMENDATIONS

A general consensus reached by both subsections
pointed to our incomplete understanding of the distribution
(Carriker, 1951; Wood and Hargis, 1971; Andrews, 1983) and
behavior (Carriker, 1 986) of different larval stages of the oyster
within estuaries. In addition, the uniqueness of the circula-
tion patterns of different estuarine bodies of water (Pritchard,
1955) and the effects of these patterns on larval dispersal
(Pritchard, 1953; Wood and Hargis, 1971) was also recog-
nized. Given these inadequacies in our knowledge, any pro-
posed monitoring program would be inherently flawed, and
therefore would fail to provide adequate verification of any
proposed model. Finally, problems encountered with samp-
ling, identifying larvae and processing samples (Wood and
Hargis, 1971; Mann, 1986) would make any proposed
monitoring program time consuming and expensive. Any at-
tempt to recommend a larval density standard for regulatory
monitoring would also be premature given the inadequate
understanding of larval densities and spatfall success (An-
drews, 1983; MacKenzie, 1983).

In spite of these limitations, both subsections proposed
recommendations based upon available knowledge and com-
mon sense. One subsection focused on political solutions to
the problem, urging a willingness for cooperation and pro-
viding suggestions for making decisions on restrictions based
on the biological information at hand. The second subsection
proposed a simple numerical model of oyster larval mortality
based on a set of conservative assumptions about larval
behavior, distribution and the dredging process.

COOPERATIVE COORDINATION

The basis of the problem faced by the Corps of
Engineers and State and Federal agencies is as follows.
Corps districts are being subjected to budgetary restraints
forcing them to look for ways to reduce dredging costs. One
way to lower these costs is to dredge during the warmer
months of the year when operational delays are fewer and
working conditions generally more favorable. On the other
hand, State and Federal resource management agencies are
responsible for protecting a dwindling oyster resource and
are keen on maintaining seasonal restrictions on dredging
in the vicinity of oyster bars, particularly during periods of
reproductive activity (summer months), corresponding with
optimal dredging periods. These groups have been
understandably at odds, thereby hampering solution of the
problem.

Two approaches are proposed to ease the stalemate
and provide for cooperation in the future. The first suggests
that cooperation, rather than antagonism, be attempted.
There is a need for all parties involved to be willing to
cooperate and be reasonably flexible. The need for flexibili-
ty lies in the fact that annual variability in natural aquatic
systems, particularly estuaries, is to a great extent unpredic-
table. Any flexibility extended at one time, by any party,
should not be looked upon as setting a precedent for the

future, but simply reflects a response to the variability in the
natural system at a particular time and place. Site specificity
is inevitable and must be a major consideration.

The second approach, based on the recognition of
variability and site specificity, suggests the establishment of
a ranking system for sites to be dredged. This system could
be based on a combination of physical criteria about the site
and its perceived value as an oyster brood or settling area.
A simple high and low ranking system could then be estab-
lished on which discussions about restrictions could be
based.

In the case of low ranked sites, scheduling of dredg-
ing could be more flexible. In the case of high ranked sites,
strict seasonal restrictions could be justified and maintained;
however, even in these cases, flexibility could be possible
in the form of assigned options by using knowledge about
prior and current oyster reproductive patterns in a given area.
For example, if a site in the upper end of a bay experiences
a wet spring, poor recruitment could be expected; in this case,
relaxation of restrictions on this site might be possible for that
season only. This would allow the Corps to fulfill its mission
at a lower cost with minimal environmental impact on the site.
This degree of flexibility would require adequate lead time
and communication between the Corps and regulatory agen-
cies. Another approach might include the practice, by the
Corps, of obtaining multiple bids for a number of jobs or multi-
ple bids on the same job at different times of the year. Both
options may give the Corps, with regulatory agency input,
more flexibility in scheduling jobs.

Both approaches come from a group of biologists who
recognize our inadequate understanding of estuarine
systems. The need for flexibility, cooperation and apprecia-
tion of the constraints placed upon all parties involved is thus
crucial. The importance of maintaining a high level of com-
munication is likewise paramount. In addition to the problem
of direct entrainment of oyster larvae, regulatory agencies
are also concerned with indirect effects of dredging on oysters
as well as direct and indirect effects on other estuarine
organisms. In this case the same level of cooperation is war-
rented between parties.

A MODEL OF LARVAL ENTRAINMENT

A numerical model of oyster larval entrainment was
developed based on a set of conservative assumptions about
dimensions of bodies of water, dredging operations and lar-
val distribution. The objective was to formulate a simple, easily
applied model based on proportions as follows:

Consider a segment of a tidal waterway through which
passes a channel to be dredged having a certain width and
length. The length of the entire waterway is set here, for
simplicity, as the length of the channel to be dredged. A
dredge operating in this channel will move a certain distance
in a day. The model assumes:

a) All oyster bars within the segment of the waterway are
productive and suitable as settling sites for larvae.

b) Late stage larvae, ready to set, are the most likely
stage affected by the dredge. This larval stage is considered

CARRIKER ETAL.: WORKSHOP CONCLUSIONS AND RECOMMENDATIONS

73

to be concentrated mainly in the lower water column (Car-
riker, 1951; Wood and Hargis, 1971; Andrews, 1 983) and is
therefore more susceptible to entrainment in the cutterhead
field (McNair and Banks, 1986) which is located within 1-2
m of the bottom.

c) Late stage larvae, ready to set, which have been pro-
duced either from oyster bars within or outside the waterway,
are distributed uniformly over the bottom of the shoal area
(over both productive and non-productive areas) and the
channel, but not necessarily at equal concentrations (i.e.,
higher densities in channels) (Andrews, 1983).

d) Some late stage larvae occurring in the channel over
a bottom unsuitable for setting, would rise out of the chan-
nel and become distributed more or less uniformly over the
shoal area.

e) In the presence of dredging, late stage larvae occur-
ring in the section of the channel being dredged would be lost
from the population by entrainment.

The parameters of the model include:

Lc = total length of channel to be dredged
Led = length of channel dredged per day
Wc = width of channel to be dredged
Ww = width of segment of waterway containing dredged
channel

Td = duration of dredging in days (= Lc/Lcd)

Ts = duration of spawning season in days
Ab = area of productive oyster bars within the waterway
Aw = total area of segment of waterway containing
dredged channel

n = number of late stage larvae present per unit area
The number of larvae present over the area of channel to be
dredged per day is given by:

n • Led • Wc

The ratio of the total number of late stage larvae which, at
a given instant of time, are over the productive oyster bars,
to the total number of late stage larvae which are within the
segment of the waterway containing the dredged channel,
is given by:

Ab / Aw

Taking the term “late stage larvae” to mean larvae that are
ready to set, and would set if provided a suitable substrate,
then the ratio Ab / Aw also represents the fraction of late stage
larvae present, at a given instant of time, in the segment of
the waterway containing the dredged channel which would
set on the productive bars at that specified time. A key
assumption of this model is that the fraction of late stage lar-
vae that are present at a given instant of time, covering any
unit area within the segment of the waterway containing the
dredged channel, and which will ultimately set on produc-
tive bars, is also equal to the ratio Ab / Aw.

In the absence of dredging, late stage larvae present
in the channel would, by previous assumptions, move out of
the channel and be distributed more or less uniformly over
the subject area. The total number of late stage larvae that
would set (N set) on the productive bars over the duration
of the spawning season is given by:

N set = n • Ab • Ts • + n • Led • Wc • (Ab / Aw) • Ts

In the presence of dredging, the late stage larvae lost to the
population (N lost) over the duration of dredging activity is
given by:

N lost = n • Led • Wc • (Ab / Aw) • Td
The fraction of late stage larvae lost to the population due
to the dredging operation (F lost) is given by the ratio N lost
/ N set as follows:

p |0St _ n • Led • Wc • (Ab / Aw) • Td

n • Ab • Ts + n • Led • Wc • (Ab / Aw) • Ts
After terms are canceled out the formula becomes:

F lost = Led • Wc - Td

Aw • Ts + Led • Wc • Ts

The number of late stage larvae ready to set, however, may
not be uniformly distributed over the total area (Andrews,
1 983). In the case where more larvae per unit area are in the
channel, the term n in the numerator and in the second term
in the denominator would have to be set larger than the first
term in the denominator. For the case of differentially
distributed larvae, with m times as many in the channel as
compared to the shoal, the formula for the fraction lost
becomes:

F lost =

Aw • Ts + m • Led • Wc • Ts

The proposed model was applied to an example of a
wide and a narrow waterway assuming that the number of
late stage larvae present per unit area (n) was 200.

Example A: Wide Waterway. Assume a waterway that
is 20,000 ft (6000 m) wide (Ww) with a channel to be dredged
which is 10,000 ft (3000 m) long (Lc) and 60 ft (18 m) wide
Wc). The duration of dredging is calculated to be 50 days
(Led) assuming the dredge advances 200 ft (60 m) per day.
The duration of spawning is set at 60 days. Assuming an even
distribution of larvae over the channel and the shoal area (m
= 1), the fraction of larvae lost (F lost), in terms of percent,
is 0.005%. Assuming a waterway half as wide with twice as
many larvae in the channel as over the shoals (m = 2), the
fraction of larvae lost is 0.02%.

Example B: Narrow Waterway. Assume a waterway
that is 2,000 ft (600 m) wide (Ww) with a channel to be
dredged which is 1 ,800 ft (550 m) long (Lc) and 250 ft (75
m) wide (Wc). The duration of dredging is calculated to be
90 days (Led) assuming the dredge advances 20 ft (6 m) per
day. The duration of spawning is set at 90 days. Assuming
twice as many larvae present over the channel as over the
shoals (m = 2), the fraction of larvae lost is 0.3%.

PERSPECTIVE

The following comments address questions about the
nature and degree to which hydraulic cutterhead dredge
operations affect larval oyster populations in general. Con-
clusions should apply logically to any geographic situation,
and thus to all estuaries, and are not intended to reflect con-
ditions in Chesapeake Bay alone.

A major question arising from the workshop is: what
is the ratio of dredge-induced mortality to overall oyster mor-
tality? Taking the results of the application of the proposed

74

LARVAL ENTRAINMENT

entrainment model as conservative, and therefore, “worst
case scenarios” (0.005 - 0.3%), one could conclude that
dredging imposes minimal direct mortality on the late stage
larval population. A numerical model proposed by Carter
(1986), however, predicts much higher rates of dredge-
induced mortality. The model is based on a much larger set
of assumptions about age composition, sex ratio, fecundity,
spawning and larval distribution and includes all larval stages.
Carter also assumes that late stage larvae are likely to be
concentrated in channel areas within the bottom 2 meters
of the water column where they are more likely to be en-
trained. The concentration of late stage larvae in this near
bottom layer in the channel is a particularly sensitive
parameter in Carter’s model. Predicted dredge-induced
reductions in survival ranged from 1 2.6 to 55.4% depending
on when, during their larval life, the larvae are exposed to
dredging and the duration of exposure. The model proposed
here involves the use of ratios of the various parameters and
thus avoids the necessity of making assumptions concern-
ing the absolute values of these parameters.

In spite of the low dredge-induced mortality predicted
by the proposed model, there could be cases in which dimen-
sions of the body of water being dredged, depth of the water
column or densities of late stage larvae are such that larger
percentages could be lost. For example, shallow sites are
often cited as important settling areas and concerns are also
warranted when dredging activities could intercept larvae
destined for upstream settling areas. In areas where water
depth is very shallow (2-4 meters), mortality of earlier stage
larvae may become a major consideration not covered by the
present model but discussed by Carter (1986). Again, condi-
tions at specific sites must be considered.

Indirect effects of dredging on oyster settling were also
discussed, particularly the effect of silt (Carriker, 1 986). Just
how setting is inhibited by sediment remains unclear. Levels
of resuspended sediment (within 250 m of the dredge) and
subsequent siltation associated with hydraulic dredging (<
4 mm thick layer of silt) (Lunz and LaSalle, 1986) are well
within naturally occurring ranges for estuaries. It may be dif-
ficult to separate the effects of suspended sediment resulting
from dredging from other sources. As pointed out by Carriker
(1986) oysters have evolved in estuaries over geologic time
under fluctuating concentrations of suspended particles and
are thus probably capable of tolerating a wide range of such
conditions, but the specific range of levels of tolerance is
undetermined. The seasonal timing of dredging could be an
important factor in reducing the deleterious effect of dredged
suspended sediment on oyster larvae and spat. Levels of
dissolved oxygen demand in the water column and com-
pounds released from resuspended sediments are also

typically low and remain for only short durations (Lunz and
LaSalle, 1986). The range of tolerance of oyster larvae to
these several factors is undetermined and their effects can
be difficult to separate from that of other sources.

Presently, our understanding of oyster larval biology
and direct and/or indirect effects on oyster larvae is limited
and in need of furthur research. This realization is reflected
in assumptions of the proposed mode! which, hopefully, will
serve as a basis for testing and further elaboration of the sub-
ject of oyster larval biology in general.

ACKNOWLEDGMENTS

We thank Victor S. Kennedy (Horn Point Laboratories, Univ.
of Maryland) and John D. Lunz (U.S. Army Engineer Waterways Ex-
periment Station) for their comments on early drafts of the manuscript.

LITERATURE CITED

Andrews, J. D. 1983. Transport of bivalve larvae in James River,
Virginia. Journal of Shellfish Research 3:29-40.

Carriker, M. R. 1951. Ecological observations on the distribution of
oyster larvae in New Jersey estuaries. Ecological Monographs
21:19-38.

Carriker, M. R. 1986. Influence of suspended particles on biology
of oyster larvae in estuaries. American Malacological Bulletin
Special Edition No. 3:41-49.

Carter, W. R. III. 1986. An argument for retaining periods of non-
dredging for the protection of oyster resources in upper
Chesapeake Bay. American Malacological Bulletin Special Edi-
tion No. 3:5-10.

Lunz, J. D. and M. W. LaSalle. 1986. Physicochemical alterations
of the environment associated with hydraulic cutterhead
dredging. American Malacological Bulletin Special Edition No.
3:31-36.

Mackenzie, C. L., Jr. 1983. To increase oyster production in the north-
ern United States. Marine Fisheries Review 45:1-22.

Mann, R. In Press. Sampling of bivalve larvae. In: North Pacific
Workshop on Stock Assessment and Management of In-
vertebrates. G. S. Jamieson and N. Bourne, eds., Canadian
Special Publication, Fisheries and Aquatic Sciences. No. 92.

McNair, C. and G. E. Banks. Prediction of Flow Fields near the suc-
tion of a cutterhead dredge. American Malacological Bulletin
Special Edition No. 3:37-40.

Pritchard, D. W. 1953. Distribution of oyster larvae in relation to
hydrographic conditions. Proceedings Gulf and Caribbean
Fisheries Institute 5:123-132.

Pritchard, D. W. 1955. Estuarine circulation patterns. Proceedings
of the American Society of Civil Engineers 81 : 1-1 1 .

Wood, L. and W. J. Hargis, Jr. 1971. Transport of bivalve larvae in
a tidal estuary. Proceedings of the 4th European Marine
Biological Symposium 4:29-44.

o

co

X

fM/m ° isp?

v/y' '0%/' m wi’

Y *— **

w% i

> 5 > s 'yx | * S ^

3NIAN INSTITUTION ^ NOli si I NVINOSHIIWS^ S 3 I B VU a H LI B RAR I ES^SMITHSONIAN INSTITUTION NOlinill'

— to z \ <f> X CO — to

^ z tu ,£ CO JS^ ^ /&mi§o\ to *•• ^ z

y -i

Hims^s3 1 a va a n“Ju b rar i es Smithsonian” institution N0iiniiiSNi"JNviN0SHiiwszS3 i a va a n^LI B RARI

o x^rx - 2 2 xS'x „ jSSf* o S:

\

CO

m 'XLEi^ ^ - <40?* m

to — to — CO __

ONIAN INSTITUTION NOlinillSNI NVIN0SH1IINS SBIHVaan LIBRARIES SMITHSONIAN INSTITUTION NOlinill

-y ,.. CO X tO X - .• . CO -y CO

< .v4S <: ,< s 2 s

,*•/ yTv * z h z

W § w i 8

sv ^ wyy/yy/,. z Vi»i»y ^ ’wyy/M, £

> ‘W~ s v\ >

z to ». z __

HilWSU's3iaVyan^LIBRARIES'"SMITHS0NIAN INSTITUTION NOlinillSNI NVINOSHIIWS^SB I d Vd B IT «- 1 B R A R I

—y tO ^ ^ ,,v . tO —?

^ ^Tcr*7>^ III ><v4SO /Ox LlI ><oCvXr\ -t— -^-v' • • —

\

co

c

5 “ vw^ 5 5 ^ 6

ON IAN-* INSTITUTION NOlinillSNI “’nVINOSHIIWS S3 I avaa H LI B RAR I ES^SMITHSONIAN^INSTITUTION NOlinill
r* z r~ , z r~ z r~ z

O xCvnriTJx ~ .vv O ~ Xo^s O — Xo O

^ m m | m - m

CO E CO £ CO £ CO

hums S3iavaan libraries Smithsonian institution NouniiiSNi nvinoshiiws saiavaan librari

CO z „ to _ z CO z to Z

fex I A. I JV . 1 5 . I ^tA I I ^

-1— \.V\VV^ Lv /y

> ‘ 5 S^' > 2 X^os^vz > Z

Z CO ' Z oo ••■-' Z CO z to

ONIAN INSTITUTION NOlinillSNI NVIN0SH1IWS S3iaVaai1 LIBRARIES SMITHSONIAN INSTITUTION NOlinill

- CO ^ Z \ 10 ^ X CO - to

Z ^ 00 Jk ^ CO ^ to ^ / !

- A^tff?X0A h ^ “ AW%A h y^smvA - w'?,,'///* H

— /^.

q;

<

o ^ 5 ^ 5 2 VP'

z: Zi z _i z -i z -j

HiiiNS saiavuan libraries Smithsonian institution NouniiiSNi nvinoshiiws saiavaan librar

b,

rn xcjcoc^ ^ m 5; -yojTxs^X rn ’,(iSC ^ MtOiiA^.

* to £ co ± co v- £ - 1 i/m 1 a 1 1 1

IONIAN ~ INSTITUTION NOlinillSNI NVINOSHIIWS S3IBVaan LIBRARIES, SMITHSONIAN INSTITUTION NOimilJ

z
o
co
x

z 'yy'yyy t vs****®/ z "/ymy t

5 /jW^ 2 \2iL£S-z > ^ ":.v > XiouiviX s ^ >

JHiikNs^'s 3 iavaan^L,BRAR| es^smithsonian^ institution NouniiiSNi nvinoshiiws^ss iavaan_ librar

CO — CO X CO — co 2

UJ XSN £ ><Srto5Jx ^ w co

Q >X^1I1SJ>^ Q

SONIAN^ INSTITUTION ^ NOlinillSNI ~JN\/IN0SHimSZS 3 IdVaail LIBRARIES SMITHSONIAN INSTITUTION NOlinill
1— z 1— . z r~ z ^ z

, i .m wm % «r% i mk % cm § if % § i
j £ w § , i IWI § i %»# g » §

ioshiiims^sh i aVa a ii^librar i es^smithsonian institution 1/5 N0iinniSNi_NviN0SHiiwsOTS3 iavaan libra

CO — to “ to i- T > (O

CO

^ 5 'C3SP/ “r' ' " g 2 5 2 N5tS£s*' 5

^7 ^ j «| Z „J

rHSONIANJNSTITUTION_NOIinillSNI NVINOSHlilMS S3ldVaan LIBRARIES SMITHSONIAN^INSTITUTION NOIlflJ

5 /$SXS\ e xsssx ™ 1 ,<5®*. 5 /s£I$i\ I , . v5 i ^

m ~ xiomgix m

joshiiims^sb iavaan- librari es^smithsonian- institution noiidiiisni nvinoshiiins S3 lavyan libra

%yyylM Itffcl* I

'HSONIAN INSTITUTION NOlifUllSNI NVINOSHlilMS S3 I U Vd a H_ Ll B RAR 1 ES SMITHSONIAN_INSTITUTION N0I10J

O O N'i£LLSX' _ O

ioshiiims~S3 1 a va a n “lib rar i Es2SMiTHS0NiAN“JiNSTiTUTi0N_N0iiniusNipNviN0SHiitMS_S3 i avaan libra

zr-zr-^^^-

m xj m x^vosttjx -7- m ~ V*** ^ m

to _ to — co — co

HSONIAN INSTITUTION NOlifUllSNI NVINOSHlilMS S3 I dVd 3 11 LI B R AR I ES SMITHSONIAN INSTITUTION N0I10J

2: » to z co z co Z r CO

^swt^v < a S S 2 ? /<&" Zjofcv < ^ S

'✓IP'K'OX -

A, .

s,

joshiiims^ss iavaan^ librari esc°smithsonian ^institution NoiiniusN^NViNOSHiiiNs^sa iavaan libra

to — an — co z \ co —

5 viv* w^/ q; w,y /c/ w&ik&xj K v^rui^?/ h <>\ u- vawuiww' -h \%>

5 5 o ^ X^va^S/ 5 X£5i

rHsoNiAN institution NouniiiSNi^NviNOSHims^sa i a va a n libraries Smithsonian institution noiioj

n — z r- z H 2 H S

co

m ^ Xt/uo^iX rn X' w 'XoiastiX rn ^ m

co £ — co \ z co ± co

josHips S3 1 avaan libraries Smithsonian institution NouniiiSNi nvinoshiiws S3 iavaan libra

"Z. CO 2: ... CO Z co z ...

< .Ok\' S rf A' 2: < S . < a\N

/'/?/ A,/.

- co

z !T

> 5 '\ > XSojIusSX s ^ >

z co \ z — ^ *■.. z co z CO

rHSONIAN INSTITUTION NOlifUllSNI NVINOSHlilMS S3 IBVdan LIBRARIES SMITHSONIAN INSTITUTION N0I1D.
~ co — to — _ co r; co

to

% — K

® X^iuiT^X O xo^ocx “ /r^i>' o

josHiiiMS saiavaan^LiBRARi e s'2 Smithsonian-1 institution NouniiiSNi^NviNOSHims S3 1 avaan libr;

z r~ z r- z r~ , Z 1-